The MySQL engine allows you to perform SELECT queries on data that is stored on a remote MySQL server.

Creating a Table 

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],
) ENGINE = MySQL('host:port', 'database', 'table', 'user', 'password'[, replace_query, 'on_duplicate_clause']);

See a detailed description of the CREATE TABLE query.

The table structure can differ from the original MySQL table structure:

  • 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.
  • Column types may differ from those in the original MySQL table. ClickHouse tries to cast values to the ClickHouse data types.
  • Setting external_table_functions_use_nulls defines how to handle Nullable columns. Default is true, if false - table function will not make nullable columns and will insert default values instead of nulls. This is also applicable for null values inside array data types.

Engine Parameters

  • host:port — MySQL server address.

  • database — Remote database name.

  • table — Remote table name.

  • user — MySQL user.

  • password — User password.

  • replace_query — Flag that converts INSERT INTO queries to REPLACE INTO. If replace_query=1, the query is substituted.

  • on_duplicate_clause — The ON DUPLICATE KEY on_duplicate_clause expression that is added to the INSERT query.

    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 to find which on_duplicate_clause you can use with the ON DUPLICATE KEY clause.

    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.

Simple WHERE clauses such as =, !=, >, >=, <, <= are executed on the MySQL server.

The rest of the conditions and the LIMIT sampling constraint are executed in ClickHouse only after the query to MySQL finishes.

Usage Example 

Table in MySQL:

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;
| int_id | int_nullable | float | float_nullable |
|      1 |         NULL |     2 |           NULL |
1 row in set (0,00 sec)

Table in ClickHouse, retrieving data from the MySQL table created above:

CREATE TABLE mysql_table
    `float_nullable` Nullable(Float32),
    `int_id` Int32
ENGINE = MySQL('localhost:3306', 'test', 'test', 'bayonet', '123')
SELECT * FROM mysql_table
│           ᴺᵁᴸᴸ │      1 │

See Also 

Original article

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