mysql函数取代相关子查询(Correlated subquery)(二)
where stc.teacher_id=te.teacher_id and te.school_id=2
GROUP BY stc.teacher_id,subj
) t GROUP BY tid
) t2
on t1.teacher_id=t2.tid
where school_id=2
这样这个也成了瞬时的,不过筛选teacher的条件(school_id=2)执行了两次,
如果这个条件比较耗资源,应该就更慢了