第一个是先过滤，子集与另一子集相关联
SELECT a.TT_ID,A.TT_TAG,b.* FROM (SELECT * FROM New_TopicTag WHERE TT_Tag=‘维修‘)as a INNER JOIN New_Topic as b ON a.Tpc_ID = b.Tpc_ID
第二个是先关联。再过滤。
SELECT a.TT_ID,A.TT_TAG,b.* FROM New_TopicTag AS A INNER JOIN New_Topic as b ON a.Tpc_ID = b.Tpc_ID where a.TT_Tag=‘维修‘
原文：https://www.cnblogs.com/fsp69/p/15037660.html