Syntax
------

IF search_condition THEN statement_list
  [ELSEIF search_condition THEN statement_list] ...
  [ELSE statement_list]
END IF;

Description
-----------

IF implements a basic conditional construct. If the search_condition evaluates
to true, the corresponding SQL statement list is executed. If no
search_condition matches, the statement list in the ELSE clause is executed.
Each statement_list consists of one or more statements.

URL: https://mariadb.com/kb/en/if/