Syntax Reference

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 'T', write ''T''.

Terminology

When using ksqlDB, the following terminology is used.

Stream

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.

Table

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.

STRUCT

You can read nested data, in Avro and JSON 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<FieldName FieldType, ...>

The 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 MAP, ARRAY, and STRUCT.

Note

Properties is not a valid field name.

Here's an example CREATE STREAM statement that uses a STRUCT to encapsulate a street address and a postal code:

CREATE STREAM orders (
  ROWKEY BIGINT KEY,
  address STRUCT<street VARCHAR, zip INTEGER>) WITH (...);

Access the fields in a STRUCT by using the dereference operator (->):

SELECT address->city, address->zip FROM orders;

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 STRUCT.

SELECT STRUCT(name := col0, ageInDogYears := col1*7) AS dogs FROM animals

If col0 is a string and col1 is an integer, the resulting schema is:

col0 STRUCT<name VARCHAR, ageInDogYears INTEGER>

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 timestamp in ROWTIME. By default, the implicit ROWTIME column is the timestamp of a message in a Kafka topic. Timestamps have an accuracy of one millisecond.

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 varchar and 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', write ''T''. The following examples show how to escape the ' character in SQL statements.

-- Example timestamp format: yyyy-MM-dd'T'HH:mm:ssX
CREATE STREAM TEST (ROWKEY INT KEY, ID bigint, event_timestamp VARCHAR)
  WITH (kafka_topic='test_topic',
        value_format='JSON',
        timestamp='event_timestamp',
        timestamp_format='yyyy-MM-dd''T''HH:mm:ssX');

-- Example timestamp format: yyyy.MM.dd G 'at' HH:mm:ss z
CREATE STREAM TEST (ROWKEY INT KEY, ID bigint, event_timestamp VARCHAR)
  WITH (kafka_topic='test_topic',
        value_format='JSON',
        timestamp='event_timestamp',
        timestamp_format='yyyy.MM.dd G ''at'' HH:mm:ss z');

-- Example timestamp format: hh 'o'clock' a, zzzz
CREATE STREAM TEST (ROWKEY INT KEY, ID bigint, event_timestamp VARCHAR)
  WITH (kafka_topic='test_topic',
        value_format='JSON',
        timestamp='event_timestamp',
        timestamp_format='hh ''o''clock'' a, zzzz');

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 <path-to-confluent>/bin/ksql --help.

Tip

You can search and browse your command history in the ksqlDB CLI with Ctrl-R. After pressing Ctrl-R, start typing the command or any part of the command to show an auto-complete of past commands.

NAME
        ksql - KSQL CLI

SYNOPSIS
        ksql [ --config-file <configFile> ] [ {-h | --help} ]
                [ --output <outputFormat> ]
                [ --query-row-limit <streamedQueryRowLimit> ]
                [ --query-timeout <streamedQueryTimeoutMs> ] [--] <server>

OPTIONS
        --config-file <configFile>
            A file specifying configs for Ksql and its underlying Kafka Streams
            instance(s). Refer to KSQL documentation for a list of available
            configs.

        -h, --help
            Display help information

        --output <outputFormat>
            The output format to use (either 'JSON' or 'TABULAR'; can be changed
            during REPL as well; defaults to TABULAR)

        --query-row-limit <streamedQueryRowLimit>
            An optional maximum number of rows to read from streamed queries

            This options value must fall in the following range: value >= 1


        --query-timeout <streamedQueryTimeoutMs>
            An optional time limit (in milliseconds) for streamed queries

            This options value must fall in the following range: value >= 1


        --
            This option can be used to separate command-line options from the
            list of arguments (useful when arguments might be mistaken for
            command-line options)

        <server>
            The address of the Ksql server to connect to (ex:
            http://confluent.io:9098)

            This option may occur a maximum of 1 times

ksqlDB data types

ksqlDB supports the following data types.

Primitive Types

ksqlDB supports the following primitive data types:

  • BOOLEAN
  • INTEGER or [INT]
  • BIGINT
  • DOUBLE
  • VARCHAR (or STRING)

Array

ARRAY<ElementType>

Note

The 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 CREATE STREAM 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 the [] operator passing in the index. For example, SOME_ARRAY[0] retrieves the first element from the array. For more information, see Operators.

Map

MAP<KeyType, ValueType>

Note

The 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 with key cost, or null For more information, see Operators.

Struct

STRUCT<FieldName FieldType, ...>

Note

The 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 CREATE STREAM statement by using the syntax STRUCT<FieldName FieldType, ...>. For example, 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.

Decimal

DECIMAL(Precision, Scale)

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 Precision.

Mathematical operations between DOUBLE and 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.

Constants

  • String constants are enclosed in single quotation marks and may include any unicode character (e.g. 'hello', '1.2').
  • Integer constants are represented by numbers that are not enclosed in quotation marks and do not contain decimal points (e.g. 1, 2).
  • Decimal constants are represented by a string of numbers that are no enclosed in quotation marks and contain a decimal point (e.g. 1.2, 87., .94). The type of the decimal constant will be DECIMAL(p, s) where p is the total number of numeric characters in the string and s is 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. 1E0, .42E-3).
  • Boolean constants are the unquoted strings that are exactly (case-insensitive) TRUE or FALSE.

SQL statements

  • 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: `identifier`.

For example, a record with the following unparseable column names is still usable.

{"@id": 42, "col.val": value}

Use backtick characters to reference the columns:

-- Enclose unparseable column names with backticks:
CREATE STREAM s1 (ROWKEY STRING KEY, `@id` integer, `col.val` string) 

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:

CREATE STREAM `foo-bar` (id VARCHAR) WITH (kafka_topic='foo', value_format='JSON', partitions=1);

You can use the hyphenated stream name in SQL statements by enclosing it with backticks:

INSERT INTO `foo-bar` (id) VALUES ('123');
CREATE STREAM `foo-too` AS SELECT * FROM `foo-bar`;

 Message
------------------------------------------------------------------------------------
 Stream foo-too created and running. Created by query with query ID: CSAS_foo-too_5
------------------------------------------------------------------------------------

Note

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.

Key Requirements

Message Keys

The 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 WITH clause.

Example:

CREATE TABLE users (rowkey INT KEY, registertime BIGINT, gender VARCHAR, regionid VARCHAR, userid INT)
  WITH (KAFKA_TOPIC='users', VALUE_FORMAT='JSON', KEY = 'userid');

The KEY property is optional. ksqlDB uses it as an optimization hint to determine if repartitioning can be avoided when performing aggregations and joins.

The type of the column named in the KEY property must match the type of the ROWKEY column.

Important

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:

  1. 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).
  2. KEY must be set to a column of type VARCHAR aka STRING.

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

Streams

For streams, just leave out the KEY property from the WITH clause. 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.

Tables

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:

Example:

  • 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.
-- Create a stream on the original topic
CREATE STREAM users_with_wrong_key (userid INT, username VARCHAR, email VARCHAR)
  WITH (KAFKA_TOPIC='users', VALUE_FORMAT='JSON');

-- Derive a new stream with the required key changes.
-- 1) The CAST statement converts the key to the required format.
-- 2) The PARTITION BY clause re-partitions the stream based on the new, converted key.
-- The resulting schema will be: ROWKEY INT, USERNAME STRING, EMAIL STRING
-- the userId will be stored in ROWKEY.
CREATE STREAM users_with_proper_key
  WITH(KAFKA_TOPIC='users-with-proper-key') AS
  SELECT username, email
  FROM users_with_wrong_key
  PARTITION BY userid
  EMIT CHANGES;

-- Now you can create the table on the properly keyed stream.
CREATE TABLE users_table (ROWKEY INT KEY, username VARCHAR, email VARCHAR)
  WITH (KAFKA_TOPIC='users-with-proper-key',
        VALUE_FORMAT='JSON');

-- Or, if you prefer, you can keep userId in the value of the repartitioned data
-- This enables using the more descriptive `userId` rather than ROWTIME.
CREATE STREAM users_with_proper_key_and_user_id
  WITH(KAFKA_TOPIC='users_with_proper_key_and_user_id') AS
  SELECT *
  FROM users_with_wrong_key
  PARTITION BY userid
  EMIT CHANGES;

-- Now you can create the table on the properly keyed stream.
-- queries against the table can use ROWKEY and userid interchangeably
CREATE TABLE users_table_2 (ROWKEY INT KEY, userid KEY, username VARCHAR, email VARCHAR)
  WITH (KAFKA_TOPIC='users_with_proper_key_and_user_id',
        VALUE_FORMAT='JSON',
        KEY='userid');

For more information, see Partition Data to Enable Joins.

Page last revised on: 2020-02-07


Last update: 2020-02-07