經常使用的幾種sql查詢語句

sql="select username,psw,gname,tel from (t1 left join t2 on t1.t1_id=t2.t1_id) left join t3 on t1.t1_id=t3.t1_id" select a.name,b.email from a,b where a.nameid=b.nameid and a.fenleiid=3 select * from

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