当我尝试选择和更新同一个表时，mysql出错error#1241-Operandshouldcontain1column(s)触发器是DELIMITER$$CREATETRIGGERvisitor_validationBEFOREINSERTONratingsvisitorsFOREACHROWBEGINSET@ifexists=(SELECT*FROMratingcounttracksWHEREuserid=New.vistoridANDlikedate=New.likevalidationANDcountfor=New.likeordislike);IF(@ifexists=NUL