是否无法将使用外连接的结果集作为子查询的对象是否无法将使用外连接的结果集作为子查询的对象如：select * from (select * from t1 a left join t2 b on a.id=b.id)[最优解释]select * from (select * from t1 a left join t2 b on a.id=b.id) as t --加个别名
