mysql触发器insert一直不被执行

Question

 DROP TRIGGER t_afterinsert_on_tab1 ;
CREATE TRIGGER t_afterinsert_on_tab1 
AFTER insert ON a
for each row 
BEGIN

if exists(select count(*) from a where a.tag_id=new.tag_id) THEN 
     update testhuanzhuanglie set value=new.temperature where name=new.tag_id;
else
        insert into testhuanzhuanglie(NAME,value) values(new.tag_id,new.temperature);
end if;
END

Answer 1

exists(select count(*) from a where a.tag_id=new.tag_id) 这个结果恒定为true，exists值判断null时为false，而count（*）就是没有数据也是0，
AFTER insert ON a这个要改成before