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116 lines
4.5 KiB
Markdown
116 lines
4.5 KiB
Markdown
---
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toc_priority: 4
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toc_title: MySQL
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---
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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]
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(
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name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
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name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
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...
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) ENGINE = MySQL('host:port', 'database', 'table', 'user', 'password'[, replace_query, 'on_duplicate_clause'])
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SETTINGS
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[connection_pool_size=16, ]
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[connection_max_tries=3, ]
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[connection_auto_close=true ]
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;
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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
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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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```
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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` (
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-> `int_id` INT NOT NULL AUTO_INCREMENT,
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-> `int_nullable` INT NULL DEFAULT NULL,
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-> `float` FLOAT NOT NULL,
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-> `float_nullable` FLOAT NULL DEFAULT NULL,
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-> PRIMARY KEY (`int_id`));
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Query OK, 0 rows affected (0,09 sec)
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mysql> insert into test (`int_id`, `float`) VALUES (1,2);
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Query OK, 1 row affected (0,00 sec)
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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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```
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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
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(
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`float_nullable` Nullable(Float32),
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`int_id` Int32
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)
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ENGINE = MySQL('localhost:3306', 'test', 'test', 'bayonet', '123')
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```
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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─┐
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│ ᴺᵁᴸᴸ │ 1 │
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└────────────────┴────────┘
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```
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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)
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[Original article](https://clickhouse.tech/docs/en/engines/table-engines/integrations/mysql/) <!--hide-->
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