ksqlDB SQL has similar semantics to ANSI SQL:
- Terminate SQL statements with a semicolon
- Escape single-quote characters (
') inside string literals by using two successive single quotes (
''). For example, to escape
When using ksqlDB, the following terminology is used.
A stream is an unbounded sequence of structured data ("facts"). For example, we could have a stream of financial transactions such as "Alice sent $100 to Bob, then Charlie sent $50 to Bob". Facts in a stream are immutable, which means new facts can be inserted to a stream, but existing facts can never be updated or deleted. Streams can be created from an Apache Kafka® topic or derived from an existing stream. A stream's underlying data is durably stored (persisted) within a Kafka topic on the Kafka brokers.
A table is a view of a stream, or another table, and represents a collection of evolving facts. For example, we could have a table that contains the latest financial information such as "Bob's current account balance is $150". It is the equivalent of a traditional database table but enriched by streaming semantics such as windowing. Facts in a table are mutable, which means new facts can be inserted to the table, and existing facts can be updated or deleted. Tables can be created from a Kafka topic or derived from existing streams and tables. In both cases, a table's underlying data is durably stored (persisted) within a Kafka topic on the Kafka brokers.
You can read nested data, in Avro, Protobuf, JSON, and JSON_SR
formats, by using the
STRUCT type in CREATE STREAM and CREATE TABLE
statements. You can use the
STRUCT type in these SQL statements:
- CREATE STREAM/TABLE (from a topic)
- CREATE STREAM/TABLE AS SELECT (from existing streams/tables)
- SELECT (non-persistent query)
Use the following syntax to declare nested data:
STRUCT type requires you to specify a list of fields. For each
field, you specify the field name and field type. The field type can be
any of the supported ksqlDB types, including the complex types
Properties is not a valid field name.
Here's an example CREATE STREAM statement that uses a
encapsulate a street address and a postal code:
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Access the fields in a
STRUCT by using the dereference operator
For more info, see Operators.
You can create a
STRUCT in a query by specifying the names of the columns
and expressions that construct the values, separated by commas. The following
example SELECT statement creates a schema that has a
col0 is a string and
col1 is an integer, the resulting schema is:
ksqlDB Time Units¶
The following list shows valid time units for the SIZE, ADVANCE BY, SESSION, and WITHIN clauses.
- DAY, DAYS
- HOUR, HOURS
- MINUTE, MINUTES
- SECOND, SECONDS
- MILLISECOND, MILLISECONDS
For more information, see Windows in ksqlDB Queries.
ksqlDB Timestamp Formats¶
Time-based operations, like windowing, process records according to the
ROWTIME. By default, the implicit
ROWTIME column is the
timestamp of a message in a Kafka topic. Timestamps have an accuracy of
Use the TIMESTAMP property to override
ROWTIME with the contents of
the specified column. Define the format of a record's timestamp by
using the TIMESTAMP_FORMAT property.
If you use the TIMESTAMP property but don't set TIMESTAMP_FORMAT, ksqlDB
assumes that the timestamp field is a
bigint. If you set
TIMESTAMP_FORMAT, the TIMESTAMP field must be of type
have a format that the
DateTimeFormatter Java class can parse.
If your timestamp format has embedded single quotes, you can escape them
by using two successive single quotes,
''. For example, to escape
''T''. The following examples show how to escape the
character in SQL statements.
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For more information on timestamp formats, see DateTimeFormatter.
ksqlDB CLI Commands¶
The ksqlDB CLI commands can be run after
starting the ksqlDB CLI.
You can view the ksqlDB CLI help by running
You can search and browse your command history in the ksqlDB CLI
Ctrl-R. After pressing
Ctrl-R, start typing the command or any
part of the command to show an auto-complete of past commands.
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ksqlDB data types¶
ksqlDB supports the following data types.
ksqlDB supports the following primitive data types:
DELIMITED format doesn't support arrays.
ksqlDB supports fields that are arrays of another type. All the elements in the array must be of the same type. The element type can be any valid SQL type.
You can define arrays within a
CREATE TABLE or
statement by using the syntax
ARRAY<ElementType>. For example,
ARRAY<INT> defines an array of integers.
The elements of an array are zero-indexed and can be accessed by using
 operator passing in the index. For example,
retrieves the first element from the array. For more information, see
DELIMITED format doesn't support maps.
ksqlDB supports fields that are maps. A map has a key and value type. All
of the keys must be of the same type, and all of the values must be also
be of the same type. Currently only
STRING keys are supported. The
value type can be any valid SQL type.
You can define maps within a
CREATE TABLE or
CREATE STREAM statement
by using the syntax
MAP<KeyType, ValueType>. For example,
MAP<STRING, INT> defines a map with string keys and integer values.
Access the values of a map by using the
 operator and passing in the
key. For example,
SOME_MAP['cost'] retrieves the value for the entry
null For more information, see
STRUCT<FieldName FieldType, ...>
DELIMITED format doesn't support structs.
ksqlDB supports fields that are structs. A struct represents strongly typed structured data. A struct is an ordered collection of named fields that have a specific type. The field types can be any valid SQL type.
You can define a structs within a
CREATE TABLE or
statement by using the syntax
STRUCT<FieldName FieldType, ...>. For
STRUCT<ID BIGINT, NAME STRING, AGE INT> defines a struct with
three fields, with the supplied name and type.
Access the fields of a struct by using the
-> operator. For example,
SOME_STRUCT->ID retrieves the value of the struct's
ID field. For
more information, see Operators.
ksqlDB supports fields that are numeric data types with fixed precision and scale:
- Precision is the maximum total number of decimal digits to be stored, including values to the left and right of the decimal point. The precision must be greater than 1. There is no default precision.
- Scale is the number of decimal digits to the right of the decimal points.
This number must be greater than 0 and less than or equal to the value for
Mathematical operations between
DECIMAL cause the decimal to be
converted to a double value automatically. Converting from the decimal data type
to any floating point type (
DOUBLE) may cause loss of precision.
- String constants are enclosed in single quotation marks and may include any unicode
- Integer constants are represented by numbers that are not enclosed in quotation marks
and do not contain decimal points (e.g.
- Decimal constants are represented by a string of numbers that are no enclosed in quotation
marks and contain a decimal point (e.g.
.94). The type of the decimal constant will be
pis the total number of numeric characters in the string and
sis the total number of numeric characters that appear to the right of the decimal point.
- Double constants are numeric strings represented in scientific notation (e.g.
- Boolean constants are the unquoted strings that are exactly (case-insensitive)
- SQL statements must be terminated with a semicolon (
- Statements can be spread over multiple lines.
- The hyphen character,
-, isn't supported in names for streams, tables, topics, and columns.
- Don't use quotes around stream names or table names when you CREATE them.
- Use backticks around column and source names with characters that are unparseable by ksqlDB or when you want to control case.
Quoted identifiers for source and column names¶
Quoted identifiers in column names and source names are supported. If you have
names that ksqlDB can't parse, or if you need to control the case of your
column names, enclose them in backtick characters, like this:
For example, a record with the following unparseable column names is still usable.
Use backtick characters to reference the columns:
Also, you can use backtick characters for the names of sources, like streams and tables. For example, you can create a stream name that has an embedded hyphen:
You can use the hyphenated stream name in SQL statements by enclosing it with backticks:
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By default, ksqlDB converts source and column names automatically to all capital letters. Use quoted identifiers to override this behavior and fully control your source and column names.
CREATE STREAM and
CREATE TABLE statements, which read data from
a Kafka topic into a stream or table, allow you to specify a
field/column in the Kafka message value that corresponds to the Kafka
message key by setting the
KEY property of the
KEY property is optional. ksqlDB uses it as an optimization hint to
determine if repartitioning can be avoided when performing aggregations
The type of the column named in the
KEY property must match the type of the
Don't set the KEY property, unless you have validated that your stream doesn't need to be re-partitioned for future joins. If you set the KEY property, you will need to re-partition explicitly if your record key doesn't meet partitioning requirements. For more information, see Partition Data to Enable Joins.
In either case, when setting
KEY you must be sure that both of the
following conditions are true:
- For every record, the contents of the Kafka message key must be the
same as the contents of the column set in
KEY(which is derived from a field in the Kafka message value).
KEYmust be set to a column of type
If these conditions aren't met, then the results of aggregations and joins may be incorrect. However, if your data doesn't meet these requirements, you can still use ksqlDB with a few extra steps. The following section explains how.
Table-table joins can be joined only on the
KEY field, and one-to-many
(1:N) joins aren't supported.
What To Do If Your Key Is Not Set or Is In A Different Format¶
For streams, just leave out the
KEY property from the
ksqlDB will take care of repartitioning the stream for you using the
value(s) from the
GROUP BY columns for aggregates, and the join
predicate for joins.
For tables, you can still use ksqlDB if the message key is not also present in the Kafka message value or if it is not in the required format as long as one of the following statements is true:
- The message key is a unary function of the value in the desired key column.
- It is ok for the messages in the topic to be re-ordered before being inserted into the table.
First create a stream to have ksqlDB write the message key, and then declare the table on the output topic of this stream:
- Goal: You want to create a table from a topic, which is keyed by userid of type INT.
- Problem: The required key is present as a field/column (aptly named
userid) in the message value, but the actual message key in Kafka is not set or has some other value or format.
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For more information, see Partition Data to Enable Joins.
Page last revised on: 2020-05-04