ClickHouse/docs/en/operations/table_engines/mysql.md

MySQL

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

Call format:

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

Call parameters

• host:port — Address of the MySQL server.
• database — Database name on the MySQL server.
• table — Name of the table.
• user — The MySQL User.
• password — User password.
• replace_query — Flag that sets query substitution INSERT INTO to REPLACE INTO. If replace_query=1, the query is replaced.
• 'on_duplicate_clause' — Adds the ON DUPLICATE KEY UPDATE 'on_duplicate_clause' expression to the INSERT query. For example: impression = VALUES(impression) + impression. 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.

At this time, 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.

The MySQL engine does not support the Nullable data type, so when reading data from MySQL tables, NULL is converted to default values for the specified column type (usually 0 or an empty string).