COALESCE( arg1,..., argN )

Yes

Returns the first argument passed to it that is non-NULL. Returns NULL if all arguments are NULL. Requires at least two arguments.

IFNULL( arg1, arg2 )

Yes

Returns arg1 if not NULL, otherwise arg2. Basically, IFNULL is a two-argument version of COALESCE().

NULLIF( arg1, arg2 )

Yes

Returns arg1 if arg1 and arg2 are different and returns NULL if the arguments are equal. Used when you have non-NULL values in your database that you want to treat as NULL.