OLD_PASSWORD() was added to MySQL when the implementation of
PASSWORD() was changed to improve security.
OLD_PASSWORD() returns the
value of the old (pre-MySQL 4.1) implementation of PASSWORD() as a
string, and is intended to permit you to reset passwords for any
pre-4.1 clients that need to connect to a more recent MySQL server version, or any version of MariaDB,
without locking them out.
As of MariaDB 5.5, the return value is a nonbinary string in the connection character set and collation, determined by the values of the character_set_connection and collation_connection system variables. Before 5.5, the return value was a binary string.
The return value is 16 bytes in length, or NULL if the argument was NULL.