左连接查询的思考

左连接查询

select uGroup_IduGroup_NamelUser_IdlPosition_Id

from usergroup u

left join (select * from UserGroupLink where User_Id=admin and company_id=) as l on uGroup_Id=lGroup_Id;

as可以省略

注意写成以下这样就不可以了

select uGroup_IduGroup_NamelUser_IdlPosition_Id

from usergroup u

left join select * from (select * from UserGroupLink where User_Id=admin and company_id=) as l on uGroup_Id=lGroup_Id;

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