两表联查 先联查好还是先where好

 

 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'