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 on_duplicate_clause expression 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 MySQL documentation to find which on_duplicate_clause you can use with 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.

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).

Original article