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---
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toc_priority: 4
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toc_title: MySQL
---
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# MySQL {#mysql}
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The MySQL engine allows you to perform `SELECT` and `INSERT` queries on data that is stored on a remote MySQL server.
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## Creating a Table {#creating-a-table}
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``` sql
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CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
...
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) ENGINE = MySQL('host:port', 'database', 'table', 'user', 'password'[, replace_query, 'on_duplicate_clause'])
SETTINGS
[connection_pool_size=16, ]
[connection_max_tries=3, ]
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[connection_wait_timeout=5, ] /* 0 -- do not wait */
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[connection_auto_close=true ]
;
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```
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See a detailed description of the [CREATE TABLE ](../../../sql-reference/statements/create/table.md#create-table-query ) query.
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The table structure can differ from the original MySQL table structure:
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- Column names should be the same as in the original MySQL table, but you can use just some of these columns and in any order.
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- Column types may differ from those in the original MySQL table. ClickHouse tries to [cast ](../../../engines/database-engines/mysql.md#data_types-support ) values to the ClickHouse data types.
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- The [external_table_functions_use_nulls ](../../../operations/settings/settings.md#external-table-functions-use-nulls ) setting defines how to handle Nullable columns. Default value: 1. If 0, the table function does not make Nullable columns and inserts default values instead of nulls. This is also applicable for NULL values inside arrays.
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**Engine Parameters**
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- `host:port` — MySQL server address.
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- `database` — Remote database name.
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- `table` — Remote table name.
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- `user` — MySQL user.
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- `password` — User password.
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- `replace_query` — Flag that converts `INSERT INTO` queries to `REPLACE INTO` . If `replace_query=1` , the query is substituted.
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- `on_duplicate_clause` — The `ON DUPLICATE KEY on_duplicate_clause` expression that is added to the `INSERT` query.
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Example: `INSERT INTO t (c1,c2) VALUES ('a', 2) ON DUPLICATE KEY UPDATE c2 = c2 + 1` , where `on_duplicate_clause` is `UPDATE c2 = c2 + 1` . See the [MySQL documentation ](https://dev.mysql.com/doc/refman/8.0/en/insert-on-duplicate.html ) to find which `on_duplicate_clause` you can use with the `ON DUPLICATE KEY` clause.
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To specify `on_duplicate_clause` you need to pass `0` to the `replace_query` parameter. If you simultaneously pass `replace_query = 1` and `on_duplicate_clause` , ClickHouse generates an exception.
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Simple `WHERE` clauses such as `=, !=, >, >=, <, <=` are executed on the MySQL server.
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The rest of the conditions and the `LIMIT` sampling constraint are executed in ClickHouse only after the query to MySQL finishes.
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Supports multiple replicas that must be listed by `|` . For example:
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```sql
CREATE TABLE test_replicas (id UInt32, name String, age UInt32, money UInt32) ENGINE = MySQL(`mysql{2|3|4}:3306`, 'clickhouse', 'test_replicas', 'root', 'clickhouse');
```
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## Usage Example {#usage-example}
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Table in MySQL:
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``` text
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mysql> CREATE TABLE `test` .`test` (
-> `int_id` INT NOT NULL AUTO_INCREMENT,
-> `int_nullable` INT NULL DEFAULT NULL,
-> `float` FLOAT NOT NULL,
-> `float_nullable` FLOAT NULL DEFAULT NULL,
-> PRIMARY KEY (`int_id`));
Query OK, 0 rows affected (0,09 sec)
mysql> insert into test (`int_id`, `float` ) VALUES (1,2);
Query OK, 1 row affected (0,00 sec)
mysql> select * from test;
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+------+----------+-----+----------+
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| int_id | int_nullable | float | float_nullable |
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+------+----------+-----+----------+
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| 1 | NULL | 2 | NULL |
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+------+----------+-----+----------+
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1 row in set (0,00 sec)
```
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Table in ClickHouse, retrieving data from the MySQL table created above:
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``` sql
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CREATE TABLE mysql_table
(
`float_nullable` Nullable(Float32),
`int_id` Int32
)
ENGINE = MySQL('localhost:3306', 'test', 'test', 'bayonet', '123')
```
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``` sql
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SELECT * FROM mysql_table
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```
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``` text
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┌─float_nullable─┬─int_id─┐
│ ᴺᵁᴸᴸ │ 1 │
└────────────────┴────────┘
```
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## Settings {#mysql-settings}
Default settings are not very efficient, since they do not even reuse connections. These settings allow you to increase the number of queries run by the server per second.
### connection_auto_close {#connection-auto-close}
Allows to automatically close the connection after query execution, i.e. disable connection reuse.
Possible values:
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- 1 — Auto-close connection is allowed, so the connection reuse is disabled
- 0 — Auto-close connection is not allowed, so the connection reuse is enabled
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Default value: `1` .
### connection_max_tries {#connection-max-tries}
Sets the number of retries for pool with failover.
Possible values:
- Positive integer.
- 0 — There are no retries for pool with failover.
Default value: `3` .
### connection_pool_size {#connection-pool-size}
Size of connection pool (if all connections are in use, the query will wait until some connection will be freed).
Possible values:
- Positive integer.
Default value: `16` .
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## See Also {#see-also}
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- [The mysql table function ](../../../sql-reference/table-functions/mysql.md )
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- [Using MySQL as a source of external dictionary ](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-sources.md#dicts-external_dicts_dict_sources-mysql )