CREATE TABLE AS SELECT

Synopsis¶

CREATE [OR REPLACE] TABLE table_name
  [WITH ( property_name = expression [, ...] )]
  AS SELECT  select_expr [, ...]
  FROM from_item
  [[ LEFT | FULL | INNER ] JOIN [join_table | join_stream] ON join_criteria]* 
  [ WINDOW window_expression ]
  [ WHERE condition ]
  [ GROUP BY grouping_expression ]
  [ HAVING having_expression ]
  [ EMIT output_refinement ];

Description¶

Create a new materialized table view with a corresponding new Kafka sink topic, and stream the result of the query as a changelog into the topic.

ksqlDB enables a materialized view, which is a table that maintains running, aggregate calculations that are updated incrementally as new data rows arrive. Queries against materialized views are fast, and ksqlDB ensures that a key's rows appear in a single partition. For more information, see Materialized Views.

Materialized views keep only the aggregation, so the full history of view changes is stored in a changelog topic, which you can replay later to restore state, if a materialized view is lost.

Both of ksqlDB's two kinds of queries, pull and push, can fetch materialized view data from a table. Pull queries terminate in a traditional relational manner. Push queries stay alive to capture streaming changes. For more information, see Queries.

See CREATE TABLE AS SELECT in action

Joins¶

In ksqlDB, you can join streams to streams, streams to tables, and tables to tables, which means that you can join data at rest with data in motion.

Joins to streams can use any stream column. If the join criteria is not the key column of the stream, ksqlDB internally repartitions the data.

Joins to tables must use the table's PRIMARY KEY as the join criteria. Non-key joins are not supported. For more information, see Joining Collections.

Kafka guarantees the relative order of any two messages from one source partition only if they are also both in the same partition after the repartition. Otherwise, Kafka is likely to interleave messages. The use case will determine if these ordering guarantees are acceptable.

For more information on how to partition your data correctly for joins, see Partition Data to Enable Joins.

Note

Partitioning streams and tables is especially important for stateful or otherwise intensive queries. For more information, see Parallelization.
Once a table is created, you can't change the number of partitions. To change the partition count, you must drop the table and create it again.

The primary key of the resulting table is determined by the following rules, in order of priority:

If the query has a GROUP BY clause, the resulting number of primary key columns matches the number of grouping expressions. For each grouping expression:

1. If the grouping expression is a single source-column reference, the corresponding primary key column matches the name, type, and contents of the source column.

2. If the grouping expression is a reference to a field within a STRUCT-type column, the corresponding primary key column matches the name, type, and contents of the STRUCT field.

3. If the GROUP BY is any other expression, the primary key has a system-generated name, unless you provide an alias in the projection, and the key matches the type and contents of the result of the expression.
If the query has a join. For more information, see Join Synthetic Key Columns.
Otherwise, the primary key matches the name, unless you provide an alias in the projection, and type of the source table's primary key.

The projection must include all columns required in the result, including any primary key columns.

Serialization¶

For supported serialization formats, ksqlDB can integrate with the Confluent Schema Registry.

ksqlDB registers the value schema of the new table with Schema Registry automatically. The schema is registered under the subject <topic-name>-value.

ksqlDB can also use Schema Inference With ID to enable using a physical schema for data serialization.

Windowed aggregation¶

Specify the WINDOW clause to create a windowed aggregation. For more information, see Time and Windows in ksqlDB.

The WINDOW clause can only be used if the from_item is a stream and the query contains a GROUP BY clause.

Table properties¶

Use the WITH clause to specify details about your table.

Important

In join queries, property values are taken from the left-most stream or table of the join.

The WITH clause supports the following properties.

FORMAT¶

The serialization format of both the message key and value in the topic. For supported formats, see Serialization Formats.

Note

To use the Avro, Protobuf, or JSON_SR formats, you must enable Schema Registry and set ksql.schema.registry.url in the ksqlDB Server configuration file. For more information, see Configure ksqlDB for Avro, Protobuf, and JSON schemas.
The JSON format doesn't require Schema Registry to be enabled.
Avro and Protobuf field names are not case sensitive in ksqlDB. This matches the ksqlDB column name behavior.

You can't use the FORMAT property with the KEY_FORMAT or VALUE_FORMAT properties in the same CREATE TABLE AS SELECT statement.

KAFKA_TOPIC¶

The name of the Kafka topic that backs the table.

If KAFKA_TOPIC isn't set, the name of the table in upper case is used as the topic name.

KEY_FORMAT¶

The serialization format of the message key in the topic. For supported formats, see Serialization Formats.

In join queries, the KEY_FORMAT value is taken from the left-most stream or table.

You can't use the KEY_FORMAT property with the FORMAT property in the same CREATE TABLE AS SELECT statement.

KEY_SCHEMA_ID¶

The schema ID of the key schema in Schema Registry.

The schema is used for schema inference and data serialization.

For more information, see Schema Inference With Schema ID.

PARTITIONS¶

The number of partitions in the backing topic.

If PARTITIONS isn't set, the number of partitions of the input stream or table is used.

In join queries, the PARTITIONS value is taken from the left-most stream or table.

You can't change the number of partitions on an existing table. To change the partition count, you must drop the table and create it again.

REPLICAS¶

The number of replicas in the backing topic.

If REPLICAS isn't set, the number of replicas of the input stream or table is used.

In join queries, the REPLICAS value is taken from the left-most stream or table.

TIMESTAMP¶

Sets a column within the tables's schema to be used as the default source of ROWTIME for any downstream queries.

Timestamps have an accuracy of milliseconds.

Downstream queries that use time-based operations, like windowing, process records in this table based on the timestamp in this column. The column is used to set the timestamp on any records emitted to Kafka.

If not provided, the ROWTIME of the source table is used.

This doesn't affect the processing of the query that populates this table. For example, given the following statement:

CREATE TABLE foo WITH (TIMESTAMP='t2') AS
  SELECT * FROM bar
  WINDOW TUMBLING (size 10 seconds);
  EMIT CHANGES;

The window into which each row of bar is placed is determined by bar's ROWTIME, not t2.

TIMESTAMP_FORMAT¶

Use with the TIMESTAMP property to specify the type and format of the timestamp column.

If set, the TIMESTAMP column must be of type varchar and have a format that can be parsed with the Java DateTimeFormatter.
If not set, the ksqlDB timestamp column must be of type bigint or timestamp.

If your timestamp format has characters that require single quotes, escape them with successive single quotes, '', for example: 'yyyy-MM-dd''T''HH:mm:ssX'.

For more information, see Timestamp formats.

VALUE_DELIMITER¶

Set the delimiter string to use when VALUE_FORMAT is set to DELIMITED.

You can use a single character as a delimiter. The default is ','.

For space-delimited and tab-delimited values, use the special values SPACE or TAB instead of the actual space or tab characters.

VALUE_FORMAT¶

The serialization format of the message value in the topic. For supported formats, see Serialization Formats.

If VALUE_FORMAT isn't provided, the system default is used, defined by ksql.persistence.default.format.value. If the default is also not set, the statement is rejected as invalid.

You can't use the VALUE_FORMAT property with the FORMAT property in the same CREATE TABLE AS SELECT statement.

VALUE_SCHEMA_ID¶

The schema ID of the value schema in Schema Registry. The schema is used for schema inference and data serialization. For more information, see Schema Inference With Schema ID.

WRAP_SINGLE_VALUE¶

Specifies how ksqlDB deserializes the value of messages in the backing topic that contain only a single column.

If set to true, ksqlDB expects the column to have been serialized as a named column within a record.
If set to false, ksqlDB expects the column to have been serialized as an anonymous value.
If not supplied, the system default is used, defined by the ksql.persistence.wrap.single.values configuration property and defaulting to true.

Note

Be careful when you have a single-column schema where the value can be NULL, because NULL values have a special meaning in ksqlDB.
Supplying this property for formats that don't support wrapping, for example DELIMITED, or when the value schema has multiple columns, causes an error.

For more information, see Single field unwrapping.

Examples¶

-- Derive a new view from an existing table:
CREATE TABLE derived AS
  SELECT
    a,
    b,
    d
  FROM source
  WHERE A is not null
  EMIT CHANGES;

-- Derive a new view from an existing table with value serialization 
-- schema defined by VALUE_SCHEMA_ID:
CREATE TABLE derived WITH (
    VALUE_SCHEMA_ID=1
  ) AS
  SELECT
    a,
    b,
    d
  FROM source
  WHERE A is not null
  EMIT CHANGES;

-- Join a stream of play events to a songs table, windowing weekly, to create a weekly chart:
CREATE TABLE weeklyMusicCharts AS
   SELECT
      s.songName,
      count(1) AS playCount
   FROM playStream p
      JOIN songs s ON p.song_id = s.id
   WINDOW TUMBLING (7 DAYS)
   GROUP BY s.songName
   EMIT CHANGES;

-- Window retention: configure the number of windows in the past that ksqlDB retains.
CREATE TABLE pageviews_per_region AS
  SELECT regionid, COUNT(*) FROM pageviews
  WINDOW HOPPING (SIZE 30 SECONDS, ADVANCE BY 10 SECONDS, RETENTION 7 DAYS, GRACE PERIOD 30 MINUTES)
  WHERE UCASE(gender)='FEMALE' AND LCASE (regionid) LIKE '%_6'
  GROUP BY regionid
  EMIT CHANGES;

-- out-of-order events: accept events for up to two hours after the window ends.
CREATE TABLE top_orders AS
  SELECT orderzip_code, TOPK(order_total, 5) FROM orders
  WINDOW TUMBLING (SIZE 1 HOUR, GRACE PERIOD 2 HOURS) 
  GROUP BY order_zipcode
  EMIT CHANGES;

Last update: 2022-08-16