When used outside stored routines, the USER, CURRENT_USER, and SESSION_USER functions all return the authorization identifier of the user that created the SQL session.
See CURRENT_USER function and SESSION_USER function for details on those functions.
SESSION_USER also always returns this value when used within stored routines.
If used within a stored routine created with EXTERNAL SECURITY DEFINER, however, USER and CURRENT_USER return the authorization identifier of the user that owns the schema of the routine. This is usually the creating user, although the database owner could be the creator as well.
For information about definer's and invoker's rights, see CREATE PROCEDURE statement or CREATE FUNCTION statement.
USER
VALUES USER