2. The BQL Language

This part describes the use of the BQL Language in SensorBee. It starts with describing the general syntax of BQL, then explain how to create the structures for data in-/output and stateful operations. After that, the general processing model and the remaining BQL query types are explained. Finally, a list of operators and functions that can be used in BQL expressions is provided.

2.1. BQL Syntax

2.1.1. Lexical Structure

BQL has been designed to be easy to learn for people who have used SQL before. While keywords and commands differ in many cases, the basic structure, set of tokens, operators etc. is the same. For example, the following is (syntactically) valid BQL input:

SELECT RSTREAM given_name, last_name FROM persons [RANGE 1 TUPLES] WHERE age > 20;

CREATE SOURCE s TYPE fluentd WITH host="example.com", port=12345;


This is a sequence of three commands, one per line (although this is not required; more than one command can be on a line, and commands can span multiple lines where required). Additionally, comments can occur in BQL input. They are effectively equivalent to whitespace.

The type of commands that can be used in BQL is described in Input/Output/State Definition and Queries. Identifiers and Keywords

Tokens such as SELECT, CREATE, or INTO in the example above are examples of keywords, that is, words that have a fixed meaning in the BQL language. The tokens persons and file are examples of identifiers. They identify names of streams, sources, or other objects, depending on the command they are used in. Therefore they are sometimes simply called “names”. Keywords and identifiers have the same lexical structure, meaning that one cannot know whether a token is an identifier or a keyword without knowing the language.

BQL identifiers and keywords must begin with a letter (a-z). Subsequent characters can be letters, underscores, or digits (0-9). Keywords and unquoted identifiers are in general case insensitive.

However, there is one important difference between SQL and BQL when it comes to “column identifiers”. In BQL, there are no “columns” with names that the user can pick herself, but “field selectors” that describe the path to a value in a JSON-like document imported from outside the system. Therefore field selectors are case-sensitive (in order to be able to deal with input of the form {"a": 1, "A": 2}) and also there is a form that allows to use special characters; see Field Selectors for details.


There is a list of reserved words that cannot be used as identifiers to avoid confusion. This list can be found at https://github.com/sensorbee/sensorbee/blob/master/core/reservedwords.go. However, this restriction does not apply to field selectors. Constants

There are multiple kinds of implicitly-typed constants in BQL: strings, decimal numbers (with and without fractional part) and booleans. Constants can also be specified with explicit types, which can enable more accurate representation and more efficient handling by the system. These alternatives are discussed in the following subsections.

String Constants

A string constant in BQL is an arbitrary sequence of characters bounded by double quotes ("), for example "This is a string". To include a double-quote character within a string constant, write two adjacent double quotes, e.g., "Dianne""s horse".

No escaping for special characters is supported at the moment, but any valid UTF-8 encoded byte sequence can be used. See the string data type reference for details.

Numeric Constants

There are two different numeric data types in BQL, int and float, representing decimal numbers without and with fractional part, respectively.

An int constant is written as


A float constant is written as


Scientific notation (1e+10) as well as Infinity and NaN cannot be used in BQL statements.

Some example of valid numerical constants:


See the type references for int and float for details.


For some operations/functions it makes a difference whether int or float is used (e.g., 2/3 is 0, but 2.0/3 is 0.666666). Be aware of that when writing constants in BQL statements.

Boolean Constants

There are two keywords for the two possible boolean values, namely true and false.

See the bool data type reference for details. Operators

An operator is a sequence of the items from the following list:


See the chapter on Operators for the complete list of operators in BQL. There are no user-defined operators at the moment. Special Characters

Some characters that are not alphanumeric have a special meaning that is different from being an operator. Details on the usage can be found at the location where the respective syntax element is described. This section only exists to advise the existence and summarize the purposes of these characters.

  • Parentheses (()) have their usual meaning to group expressions and enforce precedence. In some cases parentheses are required as part of the fixed syntax of a particular BQL command.
  • Brackets ([]) are used in Array Constructors and in Field Selectors, as well as in Stream-to-Relation Operators.
  • Curly brackets ({}) are used in Map Constructors
  • Commas (,) are used in some syntactical constructs to separate the elements of a list.
  • The semicolon (;) terminates a BQL command. It cannot appear anywhere within a command, except within a string constant or quoted identifier.
  • The colon (:) is used to separate stream names and field selectors, and within field selectors to select array slices (see Extended Descend Operators).
  • The asterisk (*) is used in some contexts to denote all the fields of a table row (see Notes on Wildcards). It also has a special meaning when used as the argument of an aggregate function, namely that the aggregate does not require any explicit parameter.
  • The period (.) is used in numeric constants and to denote descend in field selectors. Comments

A comment is a sequence of characters beginning with double dashes and extending to the end of the line, e.g.:

-- This is a standard BQL comment

C-style (multi-line) comments cannot be used. Operator Precedence

The following table shows the operator precedence in BQL:

Operator/Element Description
:: typecast
- unary minus
* / % multiplication, division, modulo
+ - addition, subtraction
(any other operator) e.g., ||
= != <> <= < >= > comparison operator
NOT logical negation
AND logical conjunction
OR logical disjunction

2.1.2. Value Expressions

Value expressions are used in a variety of contexts, such as in the target list or filter condition of the SELECT command. The expression syntax allows the calculation of values from primitive parts using arithmetic, logical, set, and other operations.

A value expression is one of the following:

  • A constant or literal value
  • A field selector
  • A row metadata reference
  • An operator invocation
  • A function call
  • An aggregate expression
  • A type cast
  • An array constructor
  • A map constructor
  • Another value expression in parentheses (used to group subexpressions and override precedence)

The first option was already discussed in Constants. The following sections discuss the remaining options. Field Selectors

In SQL, each table has a well-defined schema with columns, column names and column types. Therefore, a column name is enough to check whether that column exists, what type it has and if the type that will be extracted matches the type expected by the surrounding expression.

In BQL, each row corresponds to a JSON-like object, i.e., a map with string keys and values that have one of several data types (see Data Types and Conversions). In particular, nested maps and arrays are commonplace in the data streams used with BQL. For example, a row could look like:

{"ids": [3, 17, 21, 5],
 "dists": [
  {"other": "foo", "value": 7},
  {"other": "bar", "value": 3.5}
 "found": true}

To deal with such nested data structures, BQL uses a subset of JSON Path to address values in a row.

Basic Descend Operators

In general, a JSON Path describes a path to a certain element of a JSON document. Such a document is looked at as a rooted tree and each element of the JSON Path describes how to descend from the current node to a node one level deeper, with the start node being the root. The basic rules are:

  • If the current node is a map, then




    mean “descend to the child node with the key child_key”. The second form must be used if the key name has a non-identifier shape (e.g., contains spaces, dots, brackets or similar). It is an error if the current node is not a map. It is an error if the current node does not have such a child node.

  • If the current node is an array, then


    means “descend to the (zero-based) \(k\)-th element in the array”. Negative indices count from the end end of the array (as in Python). It is an error if the current node is not an array. It is an error if the given index is out of bounds.

The first element of a JSON Path must always be a “map access” component (since the document is always a map) and the leading dot must be omitted.

For example, ids[1] in the document given above would return 17, dists[-2].other would return foo and just dists would return the array [{"other": "foo", "value": 7}, {"other": "bar", "value": 3.5}].

Extended Descend Operators

There is limited support for array slicing and recursive descend:

  • If the current node is a map or an array, then


    returns an array of all values below the current node that have the key child_key. However, once a node with key child_key has been found, it will be returned as is, even if it may possibly itself contain that key again.

    This selector cannot be used as the first component of a JSON Path. It is an error if the current node is not a map or an array. It is not an error if there is no child element with the given key.

  • If the current node is an array, then


    returns an array of all values with the indexes in the range \([\text{start}, \text{end}-1]\). One or both of start and end can be omitted, meaning “from the first element” and “until the last element”, respectively.


    returns an array of all elements with the indexes \([\text{start}, \text{start}+\text{step}, \text{start}+2\cdot\text{step}, \cdot\cdot\cdot, \text{end}-1]\) if step is positive, or \([\text{start}, \text{start}-\text{step}, \text{start}-2\cdot\text{step}, \cdot\cdot\cdot, \text{end}+1]\) if it is negative. (This description is only true for positive indices, but in fact also negative indices can be used, again counting from the end of the array.) In general, the behavior has been implemented to be very close to Python’s list slicing.

    These selectors cannot be used as the first component of a JSON Path. It is an error if it can be decided independent of the input data that the specified values do not make sense (e.g., step is 0, or end is larger than start but step is negative), but slices that will always be empty (e.g., [2:2]) are valid. Also, if it depends on the input data whether a slice specification is valid or not (e.g., [4:-4]) it is not an error, but an empty array is returned.

  • If the slicing or recursive descend operators are followed by ordinary JSON Path operators as described before, their meaning changes to ”... for every element in the array”. For example, list[1:3].foo has the same result as [list[1].foo, list[2].foo, list[3].foo] (except that the latter would fail if list is not long enough) or a Python list comprehension such as [x.foo for x in list[1:3]]. However, it is not possible to chain multiple list-returning operators: list[1:3]..foo or foo..bar..hoge are invalid.


Given the input data

    "foo": [
        {"hoge": [
            {"a": 1, "b": 2},
            {"a": 3, "b": 4} ],
         "bar": 5},
        {"hoge": [
            {"a": 5, "b": 6},
            {"a": 7, "b": 8} ],
         "bar": 2},
        {"hoge": [
            {"a": 9, "b": 10} ],
         "bar": 8}
    "nantoka": {"x": "y"}

the following table is supposed to illustrate the effect of various JSON Path expressions.

Path Result
nantoka {"x": "y"}
nantoka.x "y"
nantoka["x"] "y"
foo[0].bar 5
foo[0].hoge[-1].a 3
["foo"][0]["hoge"][-1]["a"] 3
foo[1:2].bar [2, 8]
foo..bar [5, 2, 8]
foo..hoge[0].b [2, 6, 10] Row Metadata References

Metadata is the data that is attached to a tuple but which cannot be accessed as part of the normal row data.

Tuple Timestamp

At the moment, the only metadata that can be accessed from within BQL is a tuple’s system timestamp (the time that was set by the source that created it). This timestamp can be accessed using the ts() function. If multiple streams are joined, a stream prefix is required to identify the input tuple that is referred to, i.e.,

stream_name:ts() Operator Invocations

There are three possible syntaxes for an operator invocation:

expression  operator  expression

operator  expression

expression  operator

See the section Operators for details. Function Calls

The syntax for a function call is the name of a function, followed by its argument list enclosed in parentheses:

function_name([expression [, expression ... ]])

For example, the following computes the square root of 2:


The list of built-in functions is described in section Functions. Aggregate Expressions

An aggregate expression represents the application of an aggregate function across the rows selected by a query. An aggregate function reduces multiple inputs to a single output value, such as the sum or average of the inputs. The syntax of an aggregate expression is the following:

function_name(expression [, ... ] [ order_by_clause ])

where function_name is a previously defined aggregate and expression is any value expression that does not itself contain an aggregate expression. The optional order_by_clause is described below.

In BQL, aggregate functions can take aggregate and non-aggregate parameters. For example, the string_agg function can be called like

string_agg(name, ", ")

to return a comma-separated list of all names in the respective group. However, the second parameter is not an aggregation parameter, so for a statement like

SELECT RSTREAM string_agg(name, sep) FROM ...

sep must be mentioned in the GROUP BY clause.

For many aggregate functions (e.g., sum or avg), the order of items in the group does not matter. However, for other functions (e.g., string_agg) the user has certain expectations with respect to the order that items should be fed into the aggregate function. In this case, the order_by_clause with the syntax

ORDER BY expression [ASC | DESC] [ , expression [ASC | DESC] ... ]

can be used. The rows that are fed into the aggregate function are sorted by the values of the given expression in ascending (default) or descending mode. For example,

string_agg(first_name || " " || last_name, "," ORDER BY last_name)

will create a comma-separated list of names, ordered ascending by the last name.

See Aggregate Functions for a list of built-in aggregate functions. Type Casts

A type cast specifies a conversion from one data type to another. BQL accepts two equivalent syntaxes for type casts:

CAST(expression AS type)

When a cast is applied to a value expression, it represents a run-time type conversion. The cast will succeed only if a suitable type conversion operation has been defined, see Conversions. Array Constructors

An array constructor is an expression that builds an array value using values for its member elements. A simple array constructor consists of a left square bracket [, a list of expressions (separated by commas) for the array element values, and finally a right square bracket ]. For example:

SELECT RSTREAM [7, 2 * stream:a, true, "blue"] FROM ...

Each element of the array can have a different type. In particular, the wildcard is also allowed as an expression and will include the whole current row (i.e., a map) as an array element.


Single-element arrays of strings could also be interpreted as JSON Paths and are therefore required to have a trailing comma after their only element: ["foo",] Map Constructors

A map constructor is an expression that builds a map value using string keys and arbitrary values for its member elements. A simple map constructor consists of a left curly bracket {, a list of "key": value pairs (separated by commas) for the map elements, and finally a right curly bracket }. For example:

SELECT RSTREAM {"a_const": 7, "prod": 2 * stream:a} FROM ...

The keys must be string literals (i.e., they cannot be computed expressions); in particular they must be written using double quotes. The values can be arbitrary expressions, including a wildcard. Expression Evaluation Rules

The order of evaluation of subexpressions is not defined. In particular, the inputs of an operator or function are not necessarily evaluated left-to-right or in any other fixed order.

Furthermore, if the result of an expression can be determined by evaluating only some parts of it, then other subexpressions might not be evaluated at all. For instance, if one wrote:

true OR somefunc()

then somefunc() would (probably) not be called at all. The same would be the case if one wrote:

somefunc() OR true

Note that this is not the same as the left-to-right “short-circuiting” of Boolean operators that is found in some programming languages.

2.1.3. Calling Functions

BQL allows functions to be called using only the positional notation. In positional notation, a function call is written with its argument values in the same order as they are defined in the function declaration. Therefore, while some parameters of a function can be optional, these parameters can only be omitted at the end of the parameter list.

For example,

log(100, 2)

are both valid function calls computing the logarithm of a function. The first one uses the default value 10 for the logarithm base, the second one uses the given value 2.

2.2. Input/Output/State Definition

To process streams of data, that data needs to be imported into SensorBee and the processing results have to be exported from it. This chapter introduces input and output components in BQL. It also describes how BQL supports stateful data processing using user-defined states (UDSs).

2.2.1. Data Input

BQL inputs a stream of data using a source. A source receives data defined and generated outside SensorBee, converts that data into tuples, and finally emits tuples for further processing. This section describes how a source can be created, operated, and dropped. Creating a Source

A source can be created using the CREATE SOURCE statement.

CREATE SOURCE logs TYPE file WITH path = "access_log.jsonl";

In this example, a source named logs is created and it has the type file. The file source type has a required parameter called path. The parameter is specified in the WITH clause. Once a source is created, other components described later can read tuples from the source and compute results based on them.

When multiple parameters are required, they should be separated by commas:

CREATE SOURCE src TYPE some_type
    WITH param1 = val1, param2 = val2, param3 = val3;

Each source type has its own parameters and there is no parameter that is common to all source types.

Source types can be registered to the SensorBee server as plugins. To learn how to develop and register a source plugin, see Source Plugins.

Built-in Sources

BQL has a number of built-in source types.


The file type provides a source that inputs tuples from an existing file.


The node_statuses source periodically emits tuples with information about nodes in a topology. The status includes connected nodes, number of tuples emitted from or written to the node, and so on.


The edge_statuses source periodically emits tuples with information about each edge (a.k.a. pipe) that connects a pair of nodes. Although this information is contained in tuples emitted from node_statuses source, the edge_statuses source provides more edge-centric view of IO statuses.


The dropped_tuples emits tuples dropped from a topology. It only reports once per tuple. Tuples are often dropped from a topology because a source or a stream is not connected to any other node or a SELECT statement tries to look up a nonexistent field of a tuple. Pausing and Resuming a Source

By default, a source starts emitting tuples as soon as it is created. By adding the PAUSED keyword to the CREATE SOURCE statement, it creates a source that is paused on startup:

CREATE PAUSED SOURCE logs TYPE file WITH path = "access_log.jsonl";

The RESUME SOURCE statement makes a paused source emit tuples again:


The statement takes the name of the source to be resumed.

A source can be paused after it is created by the PAUSE SOURCE statement:


The statement also takes the name of the source to be paused.

Not all sources support PAUSE SOURCE and RESUME SOURCE statements. Issuing statements to those sources results in an error. Rewinding a Source

Some sources can be rewound, that is, they emit tuples again starting from the beginning. The REWIND SOURCE statement rewinds a source if the source supports the statement:


The statement takes the name of the source to be rewound. Issuing the statement to sources that don’t support rewinding results in an error. Dropping a Source

The DROP SOURCE statement drops (i.e. removes) a source from a topology:


The statement takes the name of the source to be dropped. Other nodes in a topology cannot refer to the source once it’s dropped. Also, nodes connected to a source may be stopped cascadingly when the source gets dropped.

2.2.2. Data Output

Results of tuple processing need to be emitted to systems or services running outside the SensorBee server in order to work with them as part of a larger system. A sink receives the results of computations performed within the SensorBee server and sends them to the outside world. This section explains how sinks are operated in BQL. Creating a Sink

A sink can be created by the CREATE SINK statement:

CREATE SINK filtered_logs TYPE file WITH path = "filtered_access_log.jsonl";

The statement is very similar to the CREATE SOURCE statement. It takes the name of the new sink, its type, and parameters. Multiple parameters can also be provided as a list separated by commas. Each sink type has its own parameters and there is no parameter that is common to all sink types.

Sink types can also be registered to the SensorBee server as plugins. To learn how to develop and register a sink plugin, see Sink Plugins.

Built-in Sinks

BQL has a number of built-in sink types.


The file type provides a sink that writes tuples to a file.


A stdout sink writes output tuples to stdout.


A uds sink passes tuples to user-defined states, which is described later. Writing Data to a Sink

The INSERT INTO statement writes data to a sink:

INSERT INTO filtered_logs FROM filtering_stream;

The statement takes the name of sink to be written and the name of a source or a stream, which will be described in following chapters. Dropping a Sink

The DROP SINK statement drops a sink from a topology:

DROP SINK filtered_logs;

The statement takes the name of the sink to be dropped. The sink cannot be accessed once it gets dropped. All INSERT INTO statements writing to the dropped sink are also stopped.

2.2.3. Stateful Data Processing

SensorBee supports user-defined states (UDSs) to perform stateful streaming data processing. Such processing includes not only aggregates such as counting but also machine learning, adaptive sampling, and so on. In natural language processing, dictionaries or configurations for tokenizers can also be considered as states.

This section describes operations involving UDSs. Use cases of UDSs are described in the tutorials and how to develop a custom UDS is explained in the server programming part. Creating a UDS

A UDS can be created using the CREATE STATE statement:

CREATE STATE age_classifier TYPE jubaclassifier_arow
    WITH label_field = "age", regularization_weight = 0.001;

This statement creates a UDS named age_classifier with the type jubaclassifier_arow. It has two parameters: label_field and regularization_weight. Each UDS type has its own parameters and there is no parameter that is common to all UDS types.

A UDS is usually used via user-defined functions (UDFs) that know about the internals of a specific UDS type. See server programming part for details. Saving a State

The SAVE STATE statement persists a UDS:

SAVE STATE age_classifier;

The statement takes the name of the UDS to be saved. After the statement is issued, SensorBee saves the state based on the given configuration. The location and the format of saved data depend on the run-time configuration and are unknown to users.

The SAVE STATE statement may take a TAG to support versioning of the saved data:

SAVE STATE age_classifier TAG initial;
-- or
SAVE STATE age_classifier TAG trained;

When the TAG clause is omitted, default will be the default tag name. Loading a State

The LOAD STATE loads a UDS that was previously saved with the SAVE STATE statement:

LOAD STATE age_classifier TYPE jubaclassifier_arow;

The statement takes the name of the UDS to be loaded and its type name.

The LOAD STATE statements may also take a TAG:

LOAD STATE age_classifier TYPE jubaclassifier_arow TAG initial;
-- or
LOAD STATE age_classifier TYPE jubaclassifier_arow TAG trained;

The UDS needs to have been saved with the specified tag before. When the TAG clause is omitted, it’s same as:

LOAD STATE age_classifier TYPE jubaclassifier_arow TAG default;

The LOAD STATE statement fails if no saved state with the given name and type exists. In that case, to avoid failure and instead create a new “empty” instance, the OR CREATE IF NOT SAVED clause can be added:

LOAD STATE age_classifier TYPE jubaclassifier_arow
        WITH label_field = "age", regularization_weight = 0.001;

If there is a saved state, this statement will load it, otherwise create a new state with the given parameters. This variant, too, can be used with the TAG clause:

LOAD STATE age_classifier TYPE jubaclassifier_arow TAG trained
        WITH label_field = "age", regularization_weight = 0.001; Dropping a State

The DROP STATE statement drops a UDS from a topology:

DROP STATE age_classifier;

The statement takes the name of the UDS to be dropped. Once a UDS is dropped, it can no longer be referred to by any statement unless it is cached somewhere.

2.3. Queries

The previous chapters described how to define data sources and sinks to communicate with the outside world. Now it is discussed how to transform the data stream from those sources and write it to the defined sinks - that is, how to actually process data.

2.3.1. Processing Model Overview

The processing model in BQL is similar to what is explained in [cql]. In this model, each tuple in a stream has the shape \((t, d)\), where \(t\) is the original timestamp and \(d\) the data contained.

In order to execute SQL-like queries, a finite set of tuples from the possibly unbounded stream, a relation, is required. In the processing step at time \(t^*\), a stream-to-relation operator \(R\) that converts a certain set of tuples in the stream to a relation \(R(t^*)\) is used. This relation is then processed with a relation-to-relation operator \(O\) that is expressed in a form very closely related to an SQL SELECT statement. Finally, a relation-to-stream operator \(S\) will emit certain rows from the output relation \(O(R(t^*))\) into the output stream, possibly taking into account the results of the previous execution step \(O(R(t^*_{\text{prev}}))\). This process is illustrated in the following figure:


This three-step pipeline is executed for each tuple, but only for one tuple at a time. Therefore, during execution there is a well-defined “current tuple”. This also means that if there is no tuple in the input stream for a long time, transformation functions will not be called.

Now the kind of stream-to-relation and relation-to-stream operators that can be used in BQL is explained. Stream-to-Relation Operators

In BQL, there are two different stream-to-relation operators, a time-based one and a tuple-based one. They are also called “window operators”, since they define a sliding window on the input stream. In terms of BQL syntax, the window operator is given after a stream name in the FROM clause within brackets and using the RANGE keyword, for example:

... FROM events [RANGE 5 SECONDS] ...
... FROM data [RANGE 10 TUPLES] ...
... FROM left [RANGE 2 SECONDS], right [RANGE 5 TUPLES] ...

From an SQL point of view, it makes sense to think of stream [RANGE window-spec] as the table to operate on.

The time-based operator is used with a certain time span \(I\) (such as 60 seconds) and at point in time \(t^*\) uses all tuples in the range \([t^*-I, t^*]\) to create the relation \(R(t^*)\).

Valid time spans are positive integer or float values, followed by the SECONDS or MILLISECONDS keyword, for example [RANGE 3.5 SECONDS] or [RANGE 200 MILLISECONDS] are valid specifications. The maximal allowed values are 86,400 for SECONDS and 86,400,000 for MILLISECONDS, i.e., the maximal window size is one day.


  • The point in time \(t^*\) is not the “current time” (however that would be defined), but it is equal to the timestamp of the current tuple. This approach means that a stream can be reprocessed with identical results independent of the system clock of some server. Also it is not necessary to worry about a delay until a tuple arrives in the system and is processed there.
  • It is assumed that the tuples in the input stream arrive in the order of their timestamps. If timestamps are out of order, the window contents are not well-defined.
  • The sizes of relations \(R(t^*_1)\) and \(R(t^*_2)\) can be different, since there may be more or less tuples in the given time span. However, there is always at least one tuple in the relation (the current one).

The tuple-based operator is used with a number \(k\) and uses the last \(k\) tuples that have arrived (or all tuples that have arrived when this number is less than \(k\)) to create the relation \(R(t^*)\). The example figure above shows a tuple-based window with \(k=3\).

Valid ranges are positive integral values, followed by the TUPLES keyword, for example [RANGE 10 TUPLES] is a valid specification. The maximal allowed value is 1,048,575.


  • The timestamps of tuples do not have any effect with this operator, they can also be out of order. Only the order in which the tuples arrived is important. (Note that for highly concurrent systems, “order” is not always a well-defined term.)
  • At the beginning of stream processing, when less than \(k\) tuples have arrived, the size of the relation will be less than \(k\). [1] As soon as \(k\) tuples have arrived, the relation size will be constant.
[1]Sometimes this leads to unexpected effects or complicated workarounds, while the cases where this is a useful behavior may be few. Therefore this behavior may change in future version. Relation-to-Stream Operators

Once a resulting relation \(O(R(t^*))\) is computed, tuples in the relation need to be output as a stream again. In BQL, there are three relation-to-stream operators, RSTREAM, ISTREAM and DSTREAM. They are also called “emit operators”, since they control how tuples are emitted into the output stream. In terms of BQL syntax, the emit operator keyword is given after the SELECT keyword, for example:


The following subsections describe how each operator works. To illustrate the effects of each operator, a visual example is provided afterwards.

RSTREAM Operator

When RSTREAM is specified, all tuples in the relation are emitted. In particular, a combination of RSTREAM with a RANGE 1 TUPLES window operator leads to 1:1 input/output behavior and can be processed by a faster execution plan than general statements.

In contrast,


emits (at most) 100 tuples for every tuple in src.

ISTREAM Operator

When ISTREAM is specified, all tuples in the relation that have not been in the previous relation are emitted. (The “I” in ISTREAM stands for “insert”.) Here, “previous” refers to the relation that was computed for the tuple just before the current tuple. Therefore the current relation can contain at most one row that was not in the previous relation and thus ISTREAM can emit at most one row in each run.

In section 4.3.2 of [streamsql], it is highlighted that for the “is contained in previous relation” check, a notion of equality is required; in particular there are various possibilities how to deal with multiple tuples that have the same value. In BQL tuples with the same value are considered equal, so that if the previous relation contains the values \(\{a, b\}\) and the current relation contains the values \(\{b, a\}\), then nothing is emitted. However, multiplicities are respected, so that if the previous relation contains the values \(\{b, a, b, a\}\) and the current relation contains \(\{a, b, a, a\}\), then one \(a\) is emitted.

As an example for a typical use case,


will drop subsequent duplicates, i.e., emit only the first occurrence of a series of tuples with identical values.

To illustrate the multiplicity counting,


will emit three times \(1\) and then nothing (because after the first three tuples processed, both the previous and the current relation always look like \(\{1, 1, 1\}\).)

DSTREAM Operator

The DSTREAM operator is very similar to ISTREAM, except that it emits all tuples in the previous relation that are not also contained in the current relation. (The “D” in DSTREAM stands for “delete”.) Just as ISTREAM, equality is computed using value comparison and multiplicity counting is used: If the previous relation contains the values \(\{a, a, b, a\}\) and the current relation contains \(\{b, b, a, a\}\), then one \(a\) is emitted.

As an example for a typical use case,


will emit only the last occurrence of a series of tuples with identical values.

To illustrate the multiplicity counting,


will never emit anything.


To illustrate the difference between the three emit operators, a concrete example shall be presented. Consider the following statement (where *STREAM is a placeholder for one of the emit operators):

SELECT *STREAM id, price FROM stream [RANGE 3 TUPLES] WHERE price < 8;

This statement just takes the id and price key-value pairs of every tuple and outputs them untransformed.

In the following table, the leftmost column shows the data of the tuple in the stream, next to that is the contents of the current window \(R(t^*)\), then the results of the relation-to-relation operator \(O(R(t^*))\). In the table below, there is the list of items that would be output by the respective emit operator.

Internal Transformations
Current Tuple’s Data Current Window \(R(t^*)\) Output Relation \(O(R(t^*))\)
  (last three tuples)  
{"id": 1, "price": 3.5} {"id": 1, "price": 3.5} {"id": 1, "price": 3.5}
{"id": 2, "price": 4.5} {"id": 1, "price": 3.5} {"id": 2, "price": 4.5} {"id": 1, "price": 3.5} {"id": 2, "price": 4.5}
{"id": 3, "price": 10.5} {"id": 1, "price": 3.5} {"id": 2, "price": 4.5} {"id": 3, "price": 10.5} {"id": 1, "price": 3.5} {"id": 2, "price": 4.5}
{"id": 4, "price": 8.5} {"id": 2, "price": 4.5} {"id": 3, "price": 10.5} {"id": 4, "price": 8.5} {"id": 2, "price": 4.5}
{"id": 5, "price": 6.5} {"id": 3, "price": 10.5} {"id": 4, "price": 8.5} {"id": 5, "price": 6.5} {"id": 5, "price": 6.5}
Emitted Tuple Data
{"id": 1, "price": 3.5} {"id": 1, "price": 3.5}  
{"id": 1, "price": 3.5} {"id": 2, "price": 4.5} {"id": 2, "price": 4.5}  
{"id": 1, "price": 3.5} {"id": 2, "price": 4.5}    
{"id": 2, "price": 4.5}   {"id": 1, "price": 3.5}

{"id": 5, "price": 6.5}

{"id": 5, "price": 6.5} {"id": 2, "price": 4.5}
[cql]Arasu et al., “The CQL Continuous Query Language: Semantic Foundations and Query Execution”, http://ilpubs.stanford.edu:8090/758/1/2003-67.pdf
[streamsql]Jain et al., “Towards a Streaming SQL Standard”, http://cs.brown.edu/~ugur/streamsql.pdf

2.3.2. Selecting and Transforming Data

In the previous section, it was explained how BQL converts stream data into relations and back. This section is about how this relational data can be selected and transformed. This functionality is exactly what SQL’s SELECT statement was designed to do, and so in BQL the SELECT syntax is mimicked as much as possible. (Some basic knowledge of what the SQL SELECT statement does is assumed.) However, as opposed to the SQL data model, BQL’s input data is assumed to be JSON-like, i.e., with varying shapes, nesting levels, and data types; therefore the BQL SELECT statement has a number of small differences to SQL’s SELECT. Overview

The general syntax of the SELECT command is

SELECT emit_operator select_list FROM table_expression;

The emit_operator is one of the operators described in Relation-to-Stream Operators. The following subsections describe the details of select_list and table_expression. Table Expressions

A table expression computes a table. The table expression contains a FROM clause that is optionally followed by WHERE, GROUP BY, and HAVING clauses:

... FROM table_list [WHERE filter_expression]
    [GROUP BY group_list] [HAVING having_expression]
The FROM Clause

The FROM clause derives a table from one or more other tables given in a comma-separated table reference list.

FROM table_reference [, table_reference [, ...]]

In SQL, each table_reference is (in the simplest possible case) an identifier that refers to a pre-defined table, e.g., FROM users or FROM names, addresses, cities are valid SQL FROM clauses.

In BQL, only streams have identifiers, so in order to get a well-defined relation, a window specifier as explained in Stream-to-Relation Operators must be added. In particular, the examples just given for SQL FROM clauses are all not valid in BQL, but the following are:


FROM names [RANGE 2 TUPLES], addresses [RANGE 1.5 SECONDS], cities [RANGE 200 MILLISECONDS]
Using Stream-Generating Functions

BQL also knows “user-defined stream-generating functions” (UDSFs) that transform a stream into another stream and can be used, for example, to output multiple output rows per input row; something that is not possible with standard SELECT features. (These are similar to “Table Functions” in PostgreSQL.) Such UDSFs can also be used in the FROM clause: Instead of using a stream’s identifier, use the function call syntax function(param, param, ...) with the UDSF name as the function name and the base stream’s identifiers as parameters (as a string, i.e., in double quotes), possibly with other parameters. For example, if there is a UDSF called duplicate that takes the input stream’s name as the first parameter and the number of copies of each input tuple as the second, this would look as follows:

FROM duplicate("products", 3) [RANGE 10 SECONDS]
Table Joins

If more than one table reference is listed in the FROM clause, the tables are cross-joined (that is, the Cartesian product of their rows is formed). The syntax table1 JOIN table2 ON (...) is not supported in BQL. The result of the FROM list is an intermediate virtual table that can then be subject to transformations by the WHERE, GROUP BY, and HAVING clauses and is finally the result of the overall table expression.

Table Aliases

A temporary name can be given to tables and complex table references to be used for references to the derived table in the rest of the query. This is called a “table alias”. To create a table alias, write

FROM table_reference AS alias

The use of table aliases is optional, but helps to shorten statements. By default, each table can be addressed using the stream name or the UDSF name, respectively. Therefore, table aliases are only mandatory if the same stream/UDSF is used multiple times in a join. Taking aliases into account, each name must uniquely refer to one table. FROM stream [RANGE 1 TUPLES], stream [RANGE 2 TUPLES] or FROM streamA [RANGE 1 TUPLES], streamB [RANGE 2 TUPLES] AS streamA are not valid, but FROM stream [RANGE 1 TUPLES] AS streamA, stream [RANGE 2 TUPLES] AS streamB and also FROM stream [RANGE 1 TUPLES], stream [RANGE 2 TUPLES] AS other are.

The WHERE Clause

The syntax of the WHERE clause is

WHERE filter_expression

where filter_expression is any expression with a boolean value. (That is, WHERE 6 is not a valid filter, but WHERE 6::bool is.)

After the processing of the FROM clause is done, each row of the derived virtual table is checked against the search condition. If the result of the condition is true, the row is kept in the output table, otherwise (i.e., if the result is false or null) it is discarded. The search condition typically references at least one column of the table generated in the FROM clause; this is not required, but otherwise the WHERE clause will be fairly useless.

As BQL does not support the table1 JOIN table2 ON (condition) syntax, any join condition must always be given in the WHERE clause.

The GROUP BY and HAVING Clauses

After passing the WHERE filter, the derived input table might be subject to grouping, using the GROUP BY clause, and elimination of group rows using the HAVING clause. They basically have the same semantics as explained in the PostgreSQL Documentation, section 7.2.3

One current limitation of BQL row grouping is that only simple columns can be used in the GROUP BY list, no complex expressions are allowed. For example, GROUP BY round(age/10) cannot be used in BQL at the moment. Select Lists

As shown in the previous section, the table expression in the SELECT command constructs an intermediate virtual table by possibly combining tables, views, eliminating rows, grouping, etc. This table is finally passed on to processing by the “select list”. The select list determines which elements of the intermediate table are actually output.

Select-List Items

As in SQL, the select list contains a number of comma-separated expressions:

SELECT emit_operator expression [, expression] [...] FROM ...

In general, items of a select list can be arbitrary Value Expressions. In SQL, tables are strictly organized in “rows” and “columns” and the most important elements in such expressions are therefore column references.

In BQL, each input tuple can be considered a “row”, but the data can also be unstructured and the notion of a “column” is not sufficient. (In fact, each row corresponds to a map object.) Therefore, BQL uses JSON Path to address data in each row. If only one table is used in the FROM clause and only top-level keys of each JSON-like row are referenced, the BQL select list looks the same as in SQL:


If the input data has the form {"a": 7, "b": "hello", "c": false}, then the output will look exactly the same. However, JSON Path allows to access nested elements as well:


If the input data has the form {"a": {"foo": {"bar": 7}}}, then the output will be {"col_0": 7}. (See paragraph Column Labels below for details on output key naming, and the section Field Selectors for details about the available syntax for JSON Path expressions.)

Table Prefixes

Where SQL uses the dot in SELECT left.a, right.b to specify the table from which to use a column, JSON Path uses the dot to describe a child relation in a single JSON element as shown above. Therefore to avoid ambiguity, BQL uses the colon (:) character to separate table and JSON Path:

SELECT RSTREAM left:foo.bar, right:hoge FROM ...

If there is just one table to select from, the table prefix can be omitted, but then it must be omitted in all expressions of the statement. If there are multiple tables in the FROM clause, then table prefixes must be used.

Column Labels

The result value of every expression in the select list will be assigned to a key in the output row. If not explicitly specified, these output keys will be "col_0", "col_1", etc. in the order the expressions were specified in the select list. However, in some cases a more meaningful output key is chosen by default, as already shown above:

  • If the expression is a single top-level key (like a), then the output key will be the same.
  • If the expression is a simple function call (like f(a)), then the output key will be the function name.
  • If the expression refers the timestamp of a tuple in a stream (using the stream:ts() syntax), then the output key will be ts.
  • If the expression is the wildcard (*), then the input will be copied, i.e., all keys from the input document will be present in the output document.

The output key can be overridden by specifying an ... AS output_key clause after an expression. For the example above,


will result in an output row that has the shape {"x": 7} instead of {"col_0": 7}. Note that it is possible to use the same column label multiple times, but in this case it is undefined which of the values with the same alias will end up in that output key.

To place values at other places than the top level of an output row map, a subset of the JSON Path syntax described in Field Selectors can be used for column labels as well. Where such a selector describes the position in a map uniquely, the value will be placed at that location. For the input data example above,

SELECT RSTREAM a.foo.bar AS x.y[3].z FROM input [RANGE 1 TUPLES];

will result in an output document with the following shape:

{"x": {"y": [null, null, null, {"z": 7}]}}

That is, a string child_key in the column label hierarchy will assume a map at the corresponding position and put the value in that map using child_key as a key; a numeric index [n] will assume an array and put the value in the n-th position, padded with NULL items before if required. Negative list indices cannot be used. Also, Extended Descend Operators cannot be used.

It is safe to assign multiple values to non-overlapping locations of an output row created this way, as shown below:

SELECT RSTREAM 7 AS x.y[3].z, "bar" AS x.foo, 17 AS x.y[0]
  FROM input [RANGE 1 TUPLES];

This will create the following output row:

{"x": {"y": [17, null, null, {"z": 7}], "foo": "bar"}}

However, as the order in which the items of the select list are processed is not defined, it is not safe to override values placed by one select list item from another select list item. For example,

SELECT RSTREAM [1, 2, 3] AS x, 17 AS x[1] ...

does not guarantee a particular output. Also, statements such as

SELECT RSTREAM 1 AS x.y, 2 AS x[1] ...

will lead to errors because x cannot be a map and an array at the same time.

Notes on Wildcards

In SQL, the wildcard (*) can be used as a shorthand expression for all columns of an input table. However, due to the strong typing in SQL’s data model, name and type conflicts can still be checked at the time the statement is analyzed. In BQL’s data model, there is no strong typing, therefore the wildcard operator must be used with a bit of caution. For example, in


if the data in the left stream looks like {"a": 1, "b": 2} and the data in the right stream looks like {"b": 3, "c": 4}, then the output document will have the keys a, b, and c, but the value of the b key is undefined.

To select all keys from only one stream, the colon notation (stream:*) as introduced above can be used.

The wildcard can be used with a column alias as well. The expression * AS foo will nest the input document under the given key foo, i.e., input {"a": 1, "b": 2} is transformed to {"foo": {"a": 1, "b": 2}}.

On the other hand, it is also possible to use the wildcard as an alias, as in foo AS *. This will have the opposite effect, i.e., it takes the contents of the foo key (which must be a map itself) and pulls them up to top level, i.e., {"foo": {"a": 1, "b": 2}} is transformed to {"a": 1, "b": 2}.

Note that any name conflicts that arise due to the use of the wildcard operator (e.g., in *, a:*, b:*, foo AS *, bar AS *) lead to undefined values in the column with the conflicting name. However, if there is an explicitly specified output key, this will always be prioritized over a key originating from a wildcard expression.

Single Input Stream
Select List Input Row Output Row
a {"a": 1, "b": 2} {"a": 1}
a, b {"a": 1, "b": 2} {"a": 1, "b": 2}
a + b {"a": 1, "b": 2} {"col_0": 3}
a, a + b {"a": 1, "b": 2} {"a": 1, "col_1": 3}
* {"a": 1, "b": 2} {"a": 1, "b": 2}
Join on Two Streams l and r
Select List Input Row (l) Input Row (r) Output Row
l:a {"a": 1, "b": 2} {"c": 3, "d": 4} {"a": 1}
l:a, r:c {"a": 1, "b": 2} {"c": 3, "d": 4} {"a": 1, "c": 3}
l:a + r:c {"a": 1, "b": 2} {"c": 3, "d": 4} {"col_0": 4}
l:* {"a": 1, "b": 2} {"c": 3, "d": 4} {"a": 1, "b": 2}
l:*, r:c AS b {"a": 1, "b": 2} {"c": 3, "d": 4} {"a": 1, "b": 3}
l:*, r:* {"a": 1, "b": 2} {"c": 3, "d": 4} {"a": 1, "b": 2, "c": 3, "d": 4}
* {"a": 1, "b": 2} {"c": 3, "d": 4} {"a": 1, "b": 2, "c": 3, "d": 4}
* {"a": 1, "b": 2} {"b": 3, "d": 4} {"a": 1, "b": (undef.), "d": 4}

2.3.3. Building Processing Pipelines

The SELECT statement as described above returns a data stream (where the transport mechanism depends on the client in use), but often an unattended processing pipeline (i.e., running on the server without client interaction) needs to set up. In order to do so, a stream can be created from the results of a SELECT query and then used afterwards like an input stream. (The concept is equivalent to that of an SQL VIEW.)

The statement used to create a stream from an SELECT statement is:

CREATE STREAM stream_name AS select_statement;

For example:


If that statement is issued correctly, subsequent statements can refer to stream_name in their FROM clauses.

If a stream thus created is no longer needed, it can be dropped using the DROP STREAM command:

DROP STREAM stream_name;

2.3.4. Expression Evaluation

To evaluate expressions outside the context of a stream, the EVAL command can be used. The general syntax is

EVAL expression;

and expression can generally be any expression, but it cannot contain references to any columns, aggregate functions or anything that only makes sense in a stream processing context.

For example, in the SensorBee Shell, the following can be done:

> EVAL "foo" || "bar";

2.4. Data Types and Conversions

This chapter describes data types defined in BQL and how their type conversion works.

2.4.1. Overview

BQL has following data types:

Type name Description Example
null Null type NULL
bool Boolean true
int 64-bit integer 12
float 64-bit floating point number 3.14
string String "sensorbee"
blob Binary large object A blob value cannot directly be written in BQL.
timestamp Datetime information in UTC A timestamp value cannot directly be written in BQL.
array Array [1, "2", 3.4]
map Map with string keys {"a": 1, "b": "2", "c": 3.4}

These types are designed to work well with JSON. They can be converted to or from JSON with some restrictions.


User defined types are not available at the moment.

2.4.2. Types

This section describes the detailed specification of each type. null

The type null only has one value: NULL, which represents an empty or undefined value.

array can contain NULL as follows:

[1, NULL, 3.4]

map can also contain NULL as its value:

    "some_key": NULL

This map is different from an empty map {} because the key "some_key" actually exists in the map but the empty map doesn’t even have a key.

NULL is converted to null in JSON. bool

The type bool has two values: true and false. In terms of a three-valued logic, NULL represents the third state, “unknown”.

true and false are converted to true and false in JSON, respectively. int

The type int is a 64-bit integer type. Its minimum value is -9223372036854775808 and its maximum value is +9223372036854775807. Using an integer value out of this range result in an error.


Due to bug #56 the current minimum value that can be parsed is actually -9223372036854775807.

An int value is converted to a number in JSON.


Some implementations of JSON use 64-bit floating point number for all numerical values. Therefore, they might not be able to handle integers greater than or equal to 9007199254740992 (i.e. 2^53) accurately. float

The type float is a 64-bit floating point type. Its implementation is IEEE 754 on most platforms but some platforms could use other implementations.

A float value is converted to a number in JSON.


Some expressions and functions may result in an infinity or a NaN. Because JSON doesn’t have an infinity or a NaN notation, they will become null when they are converted to JSON. string

The type string is similar to SQL’s type text. It may contain an arbitrary length of characters. It may contain any valid UTF-8 character including a null character.

A string value is converted to a string in JSON. blob

The type blob is a data type for any variable length binary data. There is no way to write a value directly in BQL yet, but there are some ways to use blob in BQL:

  • Emitting a tuple containing a blob value from a source
  • Casting a string encoded in base64 to blob
  • Calling a function returning a blob value

A blob value is converted to a base64-encoded string in JSON. timestamp

The type timestamp has date and time information in UTC. timestamp only guarantees precision in microseconds. There is no way to write a value directly in BQL yet, but there are some ways to use blob in BQL:

  • Emitting a tuple containing a timestamp value from a source
  • Casting a value of a type that is convertible to timestamp
  • Calling a function returning a timestamp value

A timestamp value is converted to a string in RFC3339 format with nanosecond precision in JSON: "2006-01-02T15:04:05.999999999Z07:00". Although the format can express nanoseconds, timestamp in BQL only guarantees microsecond precision as described above. array

The type array provides an ordered sequence of values of any type, for example:

[1, "2", 3.4]

An array value can also contain another array or map as a value:

    [1, "2", 3.4],
        ["4", 5.6, 7],
        [true, false, NULL],
        {"a": 10}
        "nested_array": [12, 34.5, "67"]

An array value is converted to an array in JSON. map

The type map represents an unordered set of key-value pairs. A key needs to be a string and a value can be of any type:

    "a": 1,
    "b": "2",
    "c": 3.4

A map value can contain another map or array as its value:

    "a": {
        "aa": 1,
        "ab": "2",
        "ac": 3.4
    "b": {
        "ba": {"a": 10},
        "bb": ["4", 5.6, 7],
        "bc": [true, false, NULL]
    "c": [12, 34.5, "67"]

A map is converted to an object in JSON.

2.4.3. Conversions

BQL provides a CAST(value AS type) operator, or value::type as syntactic sugar, that converts the given value to a corresponding value in the given type, if those types are convertible. For example, CAST(1 AS string), or 1::string, converts an int value 1 to a string value and results in "1". Converting to the same type as the value’s type is valid. For instance, "str"::string does not do anything and results in "str".

The following types are valid for the target type of CAST operator:

  • bool
  • int
  • float
  • string
  • blob
  • timestamp

Specifying null, array, or map as the target type results in an error.

This section describes how type conversions work in BQL.


Converting a NULL value into any type results in NULL and it is not explicitly described in the subsections. To bool

Following types can be converted to bool:

  • int
  • float
  • string
  • blob
  • timestamp
  • array
  • map
From int

0 is converted to false. Other values are converted to true.

From float

0.0, -0.0, and NaN are converted to false. Other values including infinity result in true.

From string

Following values are converted to true:

  • "t"
  • "true"
  • "y"
  • "yes"
  • "on"
  • "1"

Following values are converted to false:

  • "f"
  • "false"
  • "n"
  • "no"
  • "off"
  • "0"

Comparison is case-insensitive and leading and trailing whitespaces in a value are ignored. For example, " tRuE "::bool is true. Converting a value that is not mentioned above results in an error.

From blob

An empty blob value is converted to false. Other values are converted to true.

From timestamp

January 1, year 1, 00:00:00 UTC is converted to false. Other values are converted to true.

From array

An empty array is converted to false. Other values result in true.

From map

An empty map is converted to false. Other values result in true. To int

Following types can be converted to int:

  • bool
  • float
  • string
  • timestamp
From bool

true::int results in 1 and false::int results in 0.

From float

Converting a float value into an int value truncates the decimal part. That is, for positive numbers it results in the greatest int value less than or equal to the float value, for negative numbers it results in the smallest int value greater than or equal to the float value:

1.0::int  -- => 1
1.4::int  -- => 1
1.5::int  -- => 1
2.01::int -- => 2
(-1.0)::int  -- => -1
(-1.4)::int  -- => -1
(-1.5)::int  -- => -1
(-2.01)::int -- => -2

The conversion results in an error when the float value is out of the valid range of int values.

From string

When converting a string value into an int value, CAST operator tries to parse it as an integer value. If the string contains a float-shaped value (even if it is "1.0"), conversion fails.

"1"::int   -- => 1

The conversion results in an error when the string value contains a number that is out of the valid range of int values, or the value isn’t a number. For example, "1a"::string results in an error even though the value starts with a number.

From timestamp

A timestamp value is converted to an int value as the number of full seconds elapsed since January 1, 1970 UTC:

("1970-01-01T00:00:00Z"::timestamp)::int        -- => 0
("1970-01-01T00:00:00.123456Z"::timestamp)::int -- => 0
("1970-01-01T00:00:01Z"::timestamp)::int         -- => 1
("1970-01-02T00:00:00Z"::timestamp)::int        -- => 86400
("2016-01-18T09:22:40.123456Z"::timestamp)::int -- => 1453108960 To float

Following types can be converted to float:

  • bool
  • int
  • string
  • timestamp
From bool

true::float results in 1.0 and false::float results in 0.0.

From int

int values are converted to the nearest float values:

1::float -- => 1.0
((9000000000000012345::float)::int)::string -- => "9000000000000012288"
From string

A string value is parsed and converted to the nearest float value:

"1.1"::float   -- => 1.1
"1e-1"::float  -- => 0.1
"-1e+1"::float -- => -10.0
From timestamp

A timestamp value is converted to a float value as the number of seconds (including a decimal part) elapsed since January 1, 1970 UTC. The integral part of the result contains seconds and the decimal part contains microseconds:

("1970-01-01T00:00:00Z"::timestamp)::float        -- => 0.0
("1970-01-01T00:00:00.000001Z"::timestamp)::float -- => 0.000001
("1970-01-02T00:00:00.000001Z"::timestamp)::float -- => 86400.000001 To string

Following types can be converted to string:

  • bool
  • int
  • float
  • blob
  • timestamp
  • array
  • map
From bool

true::string results in "true", false::string results in "false".


Keep in mind that casting the string "false" back to boolean results in the true value as described above.

From int

A int value is formatted as a signed decimal integer:

1::string     -- => "1"
(-24)::string -- => "-24"
From float

A float value is formatted as a signed decimal floating point. Scientific notation is used when necessary:

1.2::string           -- => "1.2"
10000000000.0::string -- => "1e+10"
From blob

A blob value is converted to a string value encoded in base64.


Keep in mind that the blob/string conversion using CAST always involves base64 encoding/decoding. It is not possible to see the single bytes of a blob using only the CAST operator. If there is a source that emits blob data where it is known that this is actually a valid UTF-8 string (for example, JSON or XML data), the interpretation “as a string” (as opposed to “to string”) must be performed by a UDF.

From timestamp

A timestamp value is formatted in RFC3339 format with nanosecond precision: "2006-01-02T15:04:05.999999999Z07:00".

From array

An array value is formatted as a JSON array:

[1, "2", 3.4]::string -- => "[1,""2"",3.4]"
From map

A map value is formatted as a JSON object:

{"a": 1, "b": "2", "c": 3.4}::string -- => "{""a"":1,""b"":""2"",""c"":3.4}" To timestamp

Following types can be converted to timestamp:

  • int
  • float
  • string
From int

An int value to be converted to a timestamp value is assumed to have the number of seconds elapsed since January 1, 1970 UTC:

0::timestamp          -- => 1970-01-01T00:00:00Z
1::timestamp          -- => 1970-01-01T00:00:01Z
1453108960::timestamp -- => 2016-01-18T09:22:40Z
From float

An float value to be converted to a timestamp value is assumed to have the number of seconds elapsed since January 1, 1970 UTC. Its integral part should have seconds and decimal part should have microseconds:

0.0::timestamp -- => 1970-01-01T00:00:00Z
0.000001::timestamp -- => 1970-01-01T00:00:00.000001Z
86400.000001::timestamp -- => 1970-01-02T00:00:00.000001Z
From string

A string value is parsed in RFC3339 format, or RFC3339 with nanosecond precision format:

"1970-01-01T00:00:00Z"::timestamp        -- => 1970-01-01T00:00:00Z
"1970-01-01T00:00:00.000001Z"::timestamp -- => 1970-01-01T00:00:00.000001Z
"1970-01-02T00:00:00.000001Z"::timestamp -- => 1970-01-02T00:00:00.000001Z

Converting ill-formed string values to timestamp results in an error.

2.5. Operators

This chapter introduces operators used in BQL.

2.5.1. Arithmetic Operators

BQL provides the following arithmetic operators:

Operator Description Example Result
+ Addition 6 + 1 7
- Subtraction 6 - 1 5
+ Unary plus +4 4
- Unary minus -4 -4
* Multiplication 3 * 2 6
/ Division 7 / 2 3
% Modulo 5 % 3 2

All operators accept both integers and floating point numbers. Integers and floating point numbers can be mixed in a single arithmetic expression. For example, 3 + 5 * 2.5 is valid.


Unary minus operators can be applied to a value multiple times. However, each unary minus operators must be separated by a space like - - -3 because -- and succeeding characters are parsed as a comment. For example, ---3 is parsed as -- and a comment body -3.

2.5.2. String Operators

BQL provides the following string operators:

Operator Description Example Result
|| Concatenation "Hello" || ", world" "Hello, world"

|| only accepts strings and NULL. For example, "1" || 2 results in an error. When one operand is NULL, the result is also NULL. For instance, NULL || "str", "str" || NULL, and NULL || NULL result in NULL.

2.5.3. Comparison Operators

BQL provides the following comparison operators:

Operator Description Example Result
< Less than 1 < 2 true
> Greater than 1 > 2 false
<= Less than or equal to 1 <= 2 true
>= Greater than or equal to 1 >= 2 false
= Equal to 1 = 2 false
<> or != Not equal to 1 != 2 true
IS NULL Null check false IS NULL false
IS NOT NULL Non-null check false IS NOT NULL true

All comparison operators return a boolean value.

<, >, <=, and >= are only valid when

  1. either both operands are numeric values (i.e. integers or floating point numbers)
  2. or have the same type and that type is comparable.

The following types are comparable:

  • null
  • int
  • float
  • string
  • timestamp

Valid examples are as follows:

  • 1 < 2.1

    • Integers and floating point numbers can be compared.
  • "abc" > "def"

  • 1::timestamp <= 2::timestamp

  • NULL > "a"
    • This expression is valid although it always results in NULL. See NULL Comparison below.

=, <>, and != are valid for any type even if both operands have different types. When the types of operands are different, = results in false; <> and != return true. (However, integers and floating point numbers can be compared, for example 1 = 1.0 returns true.) When operands have the same type, = results in true if both values are equivalent and others return false.


Floating point values with the value NaN are treated specially as per the underlying floating point implementation. In particular, = comparison will always be false if one or both of the operands is NaN. NULL Comparison

In a three-valued logic, comparing any value with NULL results in NULL. For example, all of following expressions result in NULL:

  • 1 < NULL
  • 2 > NULL
  • "a" <= NULL
  • 3 = NULL
  • NULL <> NULL

Therefore, do not look for NULL values with expression = NULL. To check if a value is NULL or not, use IS NULL or IS NOT NULL operator. expression IS NULL operator returns true only when an expression is NULL.


[NULL] = [NULL] and {"a": NULL} = {"a": NULL} result in true although it contradict the three-valued logic. This specification is provided for convenience. Arrays or maps often have NULL to indicate that there’s no value for a specific key but the key actually exists. In other words, {"a": NULL, "b": 1} and {"b": 1} are different. Therefore, NULL in arrays and maps are compared as if it’s a regular value. Unlike NULL, comparing NaN floating point values always results in false.

2.5.4. Presence/Absence Check

In BQL, the JSON object {"a": 6, "b": NULL} is different from {"a": 6}. Therefore, when accessing b in the latter object, the result is not NULL but an error. To check whether a key is present in a map, the following operators can be used:

Operator Description Example Example Input Result
IS MISSING Absence Check b IS MISSING {"a": 6} true
IS NOT MISSING Presence Check b IS NOT MISSING {"a": 6} false

Since the presence/absence check is done before the value is actually extracted from the map, only JSON Path expressions can be used with IS [NOT] MISSING, not arbitrary expressions. For example, a + 2 IS MISSING is not a valid expression.

2.5.5. Logical Operators

BQL provides the following logical operators:

Operator Description Example Result
AND Logical and 1 < 2 AND 2 < 3 true
OR Logical or 1 < 2 OR 2 > 3 true
NOT Logical negation NOT 1 < 2 false

Logical operators also follow the three-valued logic. For example, true AND NULL and NULL OR false result in NULL.

2.6. Functions

BQL provides a number of built-in functions that are described in this chapter. Function names and meaning of parameters have been heavily inspired by PostgreSQL. However, be aware that the accepted and returned types may differ as there is no simple mapping between BQL and SQL data types. See the Function Reference for details about each function’s behavior.

2.6.1. Numeric Functions General Functions

The table below shows some common mathematical functions that can be used in BQL.

Function Description
abs(x) absolute value
cbrt(x) cube root
ceil(x) round up to nearest integer
degrees(x) radians to degrees
div(y, x) integer quotient of y/x
exp(x) exponential
floor(x) round down to nearest integer
ln(x) natural logarithm
log(x) base 10 logarithm
log(b, x) logarithm to base b
mod(y, x) remainder of y/x
pi() “π” constant
power(a, b) a raised to the power of b
radians(x) degrees to radians
round(x) round to nearest integer
sign(x) sign of the argument (-1, 0, +1)
sqrt(x) square root
trunc(x) truncate toward zero
width_bucket(x, l, r, c) bucket of x in a histogram Pseudo-Random Functions

The table below shows functions for generating pseudo-random numbers.

Function Description
random() random value in the range \(0.0 <= x < 1.0\)
setseed(x) set seed (\(-1.0 <= x <= 1.0\)) for subsequent random() calls Trigonometric Functions

Finally, the table below shows the available trigonometric functions.

Function Description
acos(x) inverse cosine
asin(x) inverse sine
atan(x) inverse tangent
cos(x) cosine
cot(x) cotangent
sin(x) sine
tan(x) tangent

2.6.2. String Functions

The table below shows some common functions for strings that can be used in BQL.

Function Description
bit_length(s) number of bits in string
btrim(s) remove whitespace from the start/end of s
btrim(s, chars) remove chars from the start/end of s
char_length(s) number of characters in s
concat(s [, ...]) concatenate all arguments
concat_ws(sep, s [, ...]) concatenate arguments s with separator
format(s, [x, ...]) format arguments using a format string
lower(s) convert s to lower case
ltrim(s) remove whitespace from the start of s
ltrim(s, chars) remove chars from the start of s
md5(s) MD5 hash of s
octet_length(s) number of bytes in s
overlay(s, r, from) replace substring
overlay(s, r, from, for) replace substring
rtrim(s) remove whitespace from the end of s
rtrim(s, chars) remove chars from the end of s
sha1(s) SHA1 hash of s
sha256(s) SHA256 hash of s
strpos(s, t) location of substring t in s
substring(s, r) extract substring matching regex r from s
substring(s, from) extract substring
substring(s, from, for) extract substring
upper(s) convert s to upper case

2.6.3. Time Functions

Function Description
distance_us(u, v) signed temporal distance from u to v in microseconds
clock_timestamp() current date and time (changes during statement execution)
now() date and time when processing of current tuple was started

2.6.4. Array Functions

Function Description
array_length(a) number of elements in an array

2.6.5. Other Scalar Functions

Function Description
coalesce(x [, ...]) return first non-null input parameter

2.6.6. Aggregate Functions

Aggregate functions compute a single result from a set of input values. The built-in normal aggregate functions are listed in the table below. The special syntax considerations for aggregate functions are explained in Aggregate Expressions.

Function Description
array_agg(x) input values, including nulls, concatenated into an array
avg(x) the average (arithmetic mean) of all input values
bool_and(x) true if all input values are true, otherwise false
bool_or(x) true if at least one input value is true, otherwise false
count(x) number of input rows for which x is not null
count(*) number of input rows
json_object_agg(k, v) aggregates name/value pairs as a map
max(x) maximum value of x across all input values
median(x) the median of all input values
min(x) minimum value of x across all input values
string_agg(x, sep) input values concatenated into a string, separated by sep
sum(x) sum of x across all input values