ClickHouse/docs/en/table_engines/mysql.md

MySQL

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

The engine takes 4 parameters: the server address (host and port); the name of the database; the name of the table; the user's name; the user's password. Example:

MySQL('host:port', 'database', 'table', 'user', 'password');

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.