??????????????????????????????????????????????????????п????????????????????????????????檔
????SELECT  *  FROM  (SELECT * FROM student  where sAge<30)  as t
??????з????????????С????????????????????????????
??????????????????????????????????=??!=??<??<=??>??>=????????????????????????????????????
tips??
select * from TblStudent
where
exists
(
--?????????????????????????е???е?tsClassid??????
select * from TblClass
wheret ClassName='??????????' and  tClassId=TblStudent.tsClassId
)
???????????????е??е???????????????????????????????????????????in????????=??
????select * from student where sClassId in  ( select cId from class where cName='??????' or cName='??????' )  ?????
????Select* from student where exists(select * from class where (cName=‘??????’or cName=‘???????’)and class.cid=student.sclassid)