SELECT IFNULL(NULL,'test'); //test
SELECT IFNULL('wang','test'); //wang
SELECT IFNULL(NULL,NULL); //NULL
SELECT COALESCE('wang','test'); //wang
SELECT COALESCE(NULL,'test'); //test
SELECT COALESCE('wang','test','test1','test2'); //wang
SELECT COALESCE(NULL,NULL,NULL,'test2'); //test2
SELECT COALESCE(NULL,NULL,NULL,NULL); //NULL