<a id="ksqldb-reference-create-stream-as-select"></a>

# CREATE STREAM AS SELECT statement in ksqlDB for Confluent Platform

## Synopsis

```sql
CREATE [OR REPLACE] STREAM stream_name
  [WITH ( property_name = expression [, ...] )]
  AS SELECT  select_expr [, ...]
  FROM from_stream
  [[ LEFT | FULL | INNER ]
      JOIN [join_table | join_stream]
          [WITHIN [<size> <timeunit> | (<before_size> <timeunit>, <after_size> <timeunit>)]
          [GRACE PERIOD <grace_size> <timeunit>]]
      ON join_criteria]*
  [ WHERE condition ]
  [PARTITION BY column_name]
  EMIT CHANGES;
```

## Description

Create a new materialized stream view with a corresponding new
Kafka sink topic, and stream the result of the query into the sink
topic. The new stream is said to be a *persistent query* and is *derived
from* the `from_stream`.

In ksqlDB, you issue a persistent query to transform one stream into
another using a SQL programming model. You derive a new stream from an
existing one by selecting and manipulating the columns you’re interested
in.

Creating a stream from a query creates a new backing topic with the
specified number of partitions, if the topic doesn’t exist already.

#### IMPORTANT
Registering a stream on a query result with `CREATE STREAM AS SELECT`
is distinct from using the
[CREATE STREAM](create-stream.md#ksqldb-reference-create-stream) statement,
which registers a stream on a Kafka source topic.

The stream metadata, like the column layout, serialization scheme, and
other information, is placed into ksqlDB’s
[command topic](../../operate-and-deploy/how-it-works.md#ksqldb-architecture-command-topic), which is its
internal cluster communication channel.

When you create a persistent query, ksqlDB assigns a generated name
based on the name you provide. For example, if you create a stream named
“pageviews_enriched”, ksqlDB might assign an ID like
“CSAS_PAGEVIEWS_ENRICHED_1”. The prepended string, “CSAS”, is an acronym
for `CREATE STREAM AS SELECT`.

ksqlDB reads rows from the stream partitions that the query selects
from. As each row passes through the persistent query, the
transformation logic is applied to create a new row. Reading a record
from Kafka doesn’t delete it. Instead, you receive a copy of it.

When you write ksqlDB programs, you chain streams and tables together.
You create a path that your data traverses, with each step in the path
performing a step of processing. ksqlDB handles the mechanics of how
your data propagates through the chain.

### Joins

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

#### IMPORTANT
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.

Joins to tables must use the table’s `PRIMARY KEY` as the join
criteria. Non-key joins aren’t supported. For more information, see
[Joining Collections](../joins/join-streams-and-tables.md#ksqldb-joins-join-event-streams).

For more information on how to partition your data correctly for joins,
see [Partition Data to Enable Joins](../joins/partition-data.md#ksqldb-joins-partition-data).

#### NOTE
- Partitioning streams and tables is especially important for stateful or
  otherwise intensive queries. For more information, see
  [Parallelization](../../operate-and-deploy/performance-guidelines.md#ksqldb-perf-guidelines-parallelization).
- Once a stream is created, you can’t change the number of partitions.
  To change the partition count, you must drop the stream and create it again.

For stream-stream joins, you must specify a `WITHIN` clause for
matching records that both occur within a specified time interval. For
valid time units, see [Time Units](../../reference/sql/time.md#ksqldb-reference-time-units).

The `PARTITION BY` clause, if supplied, is applied to the source
*after* any `JOIN` or `WHERE` clauses, and *before* the `SELECT`
clause, in much the same way as `GROUP BY`.

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

1. If the query has a `PARTITION BY` clause, the resulting number of
   key columns matches the number of expressions in the `PARTITION BY`
   clause. For each expression:
   1. If the `PARTITION BY` expression is a single source-column
      reference, the corresponding key column matches the name, type,
      and contents of the source column.
   2. If the `PARTITION BY` expression is a reference to a field
      within a `STRUCT`-type column, the corresponding key column
      matches the name, type, and contents of the `STRUCT` field.
   3. If the `PARTITION BY` expression is any other expression, the
      key column has a system-generated name, unless you provide an
      alias in the projection, and matches the type and contents of the
      result of the expression.
2. If the query has a join. For more information, see
   [Join Synthetic Key Columns](../joins/synthetic-keys.md#ksqldb-join-synthetic-key-columns).
3. Otherwise, the key matches the name, unless you provide an alias in
   the projection, and type of the source stream’s key.

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

### Serialization

For supported [serialization formats](../../reference/serialization.md#ksqldb-serialization-formats),
ksqlDB can integrate with [Confluent Schema Registry](../../../schema-registry/index.md#schemaregistry-intro).

ksqlDB registers the key and/or value schemas of the new stream with
Schema Registry automatically. Key and value schemas are registered under
the subjects `<topic-name>-key` and `<topic-name>-value`,
respectively.

ksqlDB can also use
[Schema Inference With ID](../../operate-and-deploy/schema-inference-with-id.md#ksqldb-schema-inference-with-id)
to enable using a physical schema for data serialization.

## Stream properties

Use the `WITH` clause to specify details about your stream.

#### 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](../../reference/serialization.md#ksqldb-serialization-formats).

#### NOTE
- To use the Avro, Protobuf, or JSON_SR formats, you must enable Schema Registry and set
  [ksql.schema.registry.url](../../reference/server-configuration.md#ksqldb-reference-server-configuration-schema-registry-url)
  in the ksqlDB Server configuration file. For more information, see
  [Configure ksqlDB for Avro, Protobuf, and JSON schemas](../../operate-and-deploy/installation/avro-schema.md#ksqldb-installation-configure-serialization-formats).
- 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 STREAM AS SELECT`
statement.

### KAFKA_TOPIC

The name of the Kafka topic that backs the stream.

If `KAFKA_TOPIC` isn’t set, the name of the stream 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](../../reference/serialization.md#ksqldb-serialization-formats).

If this property is not set, the format from the left-most input stream
or table is used.

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 STREAM AS SELECT` statement.

### KEY_PROTOBUF_NULLABLE_REPRESENTATION

In the default configuration, primitive fields in protobuf do not
distinguish `null` from the default values (such as zero, empty
string). To enable the use of a protobuf schema that can make this
distinction, set `KEY_PROTOBUF_NULLABLE_REPRESENTATION` to either
`OPTIONAL` or `WRAPPER`. The schema will be used to serialize keys
for the stream created by this `CREATE` statement. For more details,
see the corresponding section in the
[Serialization Formats](../../reference/serialization.md#ksqldb-serialization-formats-protobuf)
documentation.

### 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](../../operate-and-deploy/schema-inference-with-id.md#ksqldb-schema-inference-with-id).

### PARTITIONS

The number of partitions in the backing topic.

If `PARTITIONS` isn’t set, the number of partitions of the input
stream 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 stream. To
change the partition count, you must drop the stream 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.

### RETENTION_MS

The retention specified in milliseconds in the backing topic.

If `RETENTION_MS` isn’t set, the retention of the input stream is
used. But in the case of inheritance, the CREATE STREAM declaration is
not the source of the `RETENTION_MS` value.

In join queries, the `RETENTION_MS` value is taken from the left-most
stream.

You can’t change the retention on an existing stream. To change the
retention, you have these options:

- Drop the stream and the topic it’s registered on with the DROP STREAM
  and DELETE TOPIC statements, and create them again.
- Drop the stream with the DROP STREAM statement, update the topic with
  `retention.ms=<new-value>` and register the stream again with
  `CREATE STREAM WITH (RETENTION_MS=<new-value>)`.
- For a stream that was created with
  `CREATE STREAM WITH (RETENTION_MS=<old-value>)`, update the topic
  with `retention.ms=<new-value>`, and update the stream with the
  `CREATE OR REPLACE STREAM WITH (RETENTION_MS=<new-value>)`
  statement.

### TIMESTAMP

Sets a column within the stream’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 stream 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 stream is used.

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

```sql
CREATE STREAM 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](https://cnfl.io/java-dtf).
- 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](../../reference/sql/time.md#ksqldb-reference-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](../../reference/serialization.md#ksqldb-serialization-formats-protobuf).

If this property is not set, the format from the left-most input stream
or table is used.

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

You can’t use the `VALUE_FORMAT` property with the `FORMAT` property
in the same `CREATE STREAM AS SELECT` statement.

### VALUE_PROTOBUF_NULLABLE_REPRESENTATION

In the default configuration, primitive fields in protobuf do not
distinguish `null` from the default values (such as zero, empty
string). To enable the use of a protobuf schema that can make this
distinction, set `VALUE_PROTOBUF_NULLABLE_REPRESENTATION` to either
`OPTIONAL` or `WRAPPER`. The schema will be used to serialize values
for the stream created by this `CREATE` statement. For more details,
see the corresponding section in the
[Serialization Formats](../../reference/serialization.md#ksqldb-serialization-formats-protobuf) documentation.

### 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](../../operate-and-deploy/schema-inference-with-id.md#ksqldb-schema-inference-with-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](../../reference/server-configuration.md#ksqldb-reference-server-configuration-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](../../reference/serialization.md#ksqldb-serialization-formats-single-field-unwrapping).

## Examples

```sql
-- Create a view that filters an existing stream:
CREATE STREAM filtered AS
   SELECT
     a,
     few,
     columns
   FROM source_stream
   EMIT CHANGES;

-- Create a view that enriches a stream with a table lookup:
CREATE STREAM enriched AS
   SELECT
      cs.*,
      u.name,
      u.classification,
      u.level
   FROM clickstream cs
      JOIN users u ON u.id = cs.userId
   EMIT CHANGES;

-- Create a view that enriches a stream with a table lookup with value serialization schema
-- defined by VALUE_SCHEMA_ID:
CREATE STREAM enriched WITH (
    VALUE_SCHEMA_ID = 1
  ) AS
  SELECT
     cs.*,
     u.name,
     u.classification,
     u.level
  FROM clickstream cs
    JOIN users u ON u.id = cs.userId
  EMIT CHANGES;
```