ClickHouse/docs/en/query_language/create.md

CREATE DATABASE

Creates database.

CREATE DATABASE [IF NOT EXISTS] db_name [ON CLUSTER cluster] [ENGINE = engine(...)]

Clauses

• IF NOT EXISTS

  If the db_name database already exists then:

  • If clause is specified, ClickHouse doesn't create a new database and doesn't throw an exception.
  • If clause is not specified, then ClickHouse doesn't create a new database and throw and exception.

• ON CLUSTER

  ClickHouse creates the db_name database on all the servers of a specified cluster.

• ENGINE

  • MySQL

    Allows to retrieve data from the remote MySQL server.

  By default, ClickHouse uses its own database engine.

CREATE TABLE

The CREATE TABLE query can have several forms.

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [compression_codec] [TTL expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [compression_codec] [TTL expr2],
    ...
) ENGINE = engine

Creates a table named 'name' in the 'db' database or the current database if 'db' is not set, with the structure specified in brackets and the 'engine' engine. The structure of the table is a list of column descriptions. If indexes are supported by the engine, they are indicated as parameters for the table engine.

A column description is name type in the simplest case. Example: RegionID UInt32. Expressions can also be defined for default values (see below).

CREATE TABLE [IF NOT EXISTS] [db.]table_name AS [db2.]name2 [ENGINE = engine]

Creates a table with the same structure as another table. You can specify a different engine for the table. If the engine is not specified, the same engine will be used as for the db2.name2 table.

CREATE TABLE [IF NOT EXISTS] [db.]table_name AS table_fucntion()

Creates a table with the same structure and data as the value returned by table function.

CREATE TABLE [IF NOT EXISTS] [db.]table_name ENGINE = engine AS SELECT ...

Creates a table with a structure like the result of the SELECT query, with the 'engine' engine, and fills it with data from SELECT.

In all cases, if IF NOT EXISTS is specified, the query won't return an error if the table already exists. In this case, the query won't do anything.

There can be other clauses after the ENGINE clause in the query. See detailed documentation on how to create tables in the descriptions of table engines.

Default Values

The column description can specify an expression for a default value, in one of the following ways:DEFAULT expr, MATERIALIZED expr, ALIAS expr. Example: URLDomain String DEFAULT domain(URL).

If an expression for the default value is not defined, the default values will be set to zeros for numbers, empty strings for strings, empty arrays for arrays, and 0000-00-00 for dates or 0000-00-00 00:00:00 for dates with time. NULLs are not supported.

If the default expression is defined, the column type is optional. If there isn't an explicitly defined type, the default expression type is used. Example: EventDate DEFAULT toDate(EventTime) – the 'Date' type will be used for the 'EventDate' column.

If the data type and default expression are defined explicitly, this expression will be cast to the specified type using type casting functions. Example: Hits UInt32 DEFAULT 0 means the same thing as Hits UInt32 DEFAULT toUInt32(0).

Default expressions may be defined as an arbitrary expression from table constants and columns. When creating and changing the table structure, it checks that expressions don't contain loops. For INSERT, it checks that expressions are resolvable – that all columns they can be calculated from have been passed.

DEFAULT expr

Normal default value. If the INSERT query doesn't specify the corresponding column, it will be filled in by computing the corresponding expression.

MATERIALIZED expr

Materialized expression. Such a column can't be specified for INSERT, because it is always calculated. For an INSERT without a list of columns, these columns are not considered. In addition, this column is not substituted when using an asterisk in a SELECT query. This is to preserve the invariant that the dump obtained using SELECT * can be inserted back into the table using INSERT without specifying the list of columns.

ALIAS expr

Synonym. Such a column isn't stored in the table at all. Its values can't be inserted in a table, and it is not substituted when using an asterisk in a SELECT query. It can be used in SELECTs if the alias is expanded during query parsing.

When using the ALTER query to add new columns, old data for these columns is not written. Instead, when reading old data that does not have values for the new columns, expressions are computed on the fly by default. However, if running the expressions requires different columns that are not indicated in the query, these columns will additionally be read, but only for the blocks of data that need it.

If you add a new column to a table but later change its default expression, the values used for old data will change (for data where values were not stored on the disk). Note that when running background merges, data for columns that are missing in one of the merging parts is written to the merged part.

It is not possible to set default values for elements in nested data structures.

TTL expression

Defines storage time for values. Can be specified only for MergeTree-family tables. For the detailed description, see TTL for columns and tables.

Column Compression Codecs

Besides default data compression, defined in server settings, per-column specification is also available.

Supported compression algorithms:

• NONE — No compression.
• LZ4 — Lossless data compression algorithm used by default. Applies LZ4 fast compression.
• LZ4HC[(level)] — LZ4 CH (high compression) algorithm with configurable level. Default level: 9. If you set level <= 0, the default level is applied. Possible levels: [1, 12]. Recommended levels are in range: [4, 9].
• ZSTD[(level)] — ZSTD compression algorithm with configurable level. Possible levels: [1, 22]. Default value: 1.
• Delta(delta_bytes) — compression approach, when raw values are replaced with the difference of two neighbour values. Up to delta_bytes are used for storing delta value, so delta_bytes is a maximum size of raw values. Possible delta_bytes values: 1, 2, 4, 8. Default value for delta_bytes is sizeof(type), if it is equals to 1, 2, 4, 8. Otherwise it equals 1.
• DoubleDelta — Compresses values down to 1 bit (in the best case), using deltas calculation. Best compression rates are achieved on monotonic sequences with constant stride, for example, time series data. Can be used with any fixed-width type. Implements the algorithm used in Gorilla TSDB, extending it to support 64 bit types. Uses 1 extra bit for 32 byte deltas: 5 bit prefix instead of 4 bit prefix. For additional information, see the "Compressing time stamps" section of the Gorilla: A Fast, Scalable, In-Memory Time Series Database document.
• Gorilla — Compresses values down to 1 bit (in the best case). The codec is efficient when storing series of floating point values that change slowly, because the best compression rate is achieved when neighbouring values are binary equal. Implements the algorithm used in Gorilla TSDB, extending it to support 64 bit types. For additional information, see the "Compressing values" section of the Gorilla: A Fast, Scalable, In-Memory Time Series Database document.

High compression levels useful for asymmetric scenarios, like compress once, decompress a lot of times. Greater levels stands for better compression and higher CPU usage.

!!!warning You cannot decompress ClickHouse database files with external utilities, for example, lz4. Use the special utility clickhouse-compressor.

Syntax example:

CREATE TABLE codec_example
(
    dt Date CODEC(ZSTD),
    ts DateTime CODEC(LZ4HC),
    float_value Float32 CODEC(NONE),
    double_value Float64 CODEC(LZ4HC(9))
)
ENGINE = MergeTree
PARTITION BY tuple()
ORDER BY dt

Codecs can be combined in a pipeline. Default table codec is not included into pipeline (if it should be applied to a column, you have to specify it explicitly in pipeline). Example below shows an optimization approach for storing timeseries metrics. Usually, values for particular metric, stored in path does not differ significantly from point to point. Using delta-encoding allows to reduce disk space usage significantly.

CREATE TABLE timeseries_example
(
    dt Date,
    ts DateTime,
    path String,
    value Float32 CODEC(Delta, ZSTD)
)
ENGINE = MergeTree
PARTITION BY dt
ORDER BY (path, ts)

Temporary Tables

ClickHouse supports temporary tables which have the following characteristics:

• Temporary tables disappear when the session ends, including if the connection is lost.
• A temporary table use the Memory engine only.
• The DB can't be specified for a temporary table. It is created outside of databases.
• If a temporary table has the same name as another one and a query specifies the table name without specifying the DB, the temporary table will be used.
• For distributed query processing, temporary tables used in a query are passed to remote servers.

To create a temporary table, use the following syntax:

CREATE TEMPORARY TABLE [IF NOT EXISTS] table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
)

In most cases, temporary tables are not created manually, but when using external data for a query, or for distributed (GLOBAL) IN. For more information, see the appropriate sections

Distributed DDL queries (ON CLUSTER clause)

The CREATE, DROP, ALTER, and RENAME queries support distributed execution on a cluster. For example, the following query creates the all_hits Distributed table on each host in cluster:

CREATE TABLE IF NOT EXISTS all_hits ON CLUSTER cluster (p Date, i Int32) ENGINE = Distributed(cluster, default, hits)

In order to run these queries correctly, each host must have the same cluster definition (to simplify syncing configs, you can use substitutions from ZooKeeper). They must also connect to the ZooKeeper servers. The local version of the query will eventually be implemented on each host in the cluster, even if some hosts are currently not available. The order for executing queries within a single host is guaranteed. ALTER queries are not yet supported for replicated tables.

CREATE VIEW

CREATE [MATERIALIZED] VIEW [IF NOT EXISTS] [db.]table_name [TO[db.]name] [ENGINE = engine] [POPULATE] AS SELECT ...

Creates a view. There are two types of views: normal and MATERIALIZED.

Normal views don't store any data, but just perform a read from another table. In other words, a normal view is nothing more than a saved query. When reading from a view, this saved query is used as a subquery in the FROM clause.

As an example, assume you've created a view:

CREATE VIEW view AS SELECT ...

and written a query:

SELECT a, b, c FROM view

This query is fully equivalent to using the subquery:

SELECT a, b, c FROM (SELECT ...)

Materialized views store data transformed by the corresponding SELECT query.

When creating a materialized view, you must specify ENGINE – the table engine for storing data.

A materialized view is arranged as follows: when inserting data to the table specified in SELECT, part of the inserted data is converted by this SELECT query, and the result is inserted in the view.

If you specify POPULATE, the existing table data is inserted in the view when creating it, as if making a CREATE TABLE ... AS SELECT ... . Otherwise, the query contains only the data inserted in the table after creating the view. We don't recommend using POPULATE, since data inserted in the table during the view creation will not be inserted in it.

A SELECT query can contain DISTINCT, GROUP BY, ORDER BY, LIMIT... Note that the corresponding conversions are performed independently on each block of inserted data. For example, if GROUP BY is set, data is aggregated during insertion, but only within a single packet of inserted data. The data won't be further aggregated. The exception is when using an ENGINE that independently performs data aggregation, such as SummingMergeTree.

The execution of ALTER queries on materialized views has not been fully developed, so they might be inconvenient. If the materialized view uses the construction TO [db.]name, you can DETACH the view, run ALTER for the target table, and then ATTACH the previously detached (DETACH) view.

Views look the same as normal tables. For example, they are listed in the result of the SHOW TABLES query.

There isn't a separate query for deleting views. To delete a view, use DROP TABLE.

Original article