select * from (select userId from table_a where dt=20160731) a join (select userId from table_b where dt=20160731) b on = join (select userId from table_c where dt=20160731) c on =
等价于
select * from (select userId from table_a where dt=20160731) a join (select userId from table_b where dt=20160731) b on = join (select userId from table_c where dt=20160731) c on =
等价于
select * from (select userId from table_a where dt=20160731) a join (select userId from table_b where dt=20160731) b on = join (select userId from table_c where dt=20160731) c on = and =
总结：
可以把a与b表join关联看成一个新的表table_j，table_j表有两列，与c表进行关联
如果是left outer join 效果相同，只不过是将对最后一个on链接条件来说，是与table_j的第一列相连还是与第二列相连而已
等价于table_j的第二列userId()与c进行关联，不去管b表是否有能关联上
select * from(select userId from table_a where dt=20160731) a
left outer join (select userId from table_b where dt=20160731) b on =
left outer join (select userId from table_c where dt=20160731) c on =
等价于table_j的第二列userId()与c进行关联，不去管a表是否有能关联上
select * from
(select userId from table_a where dt=20160731) a
left outer join (select userId from table_b where dt=20160731) b on =
left outer join (select userId from table_c where dt=20160731) c on =
等价于table_j的第一列与第二列userId()与c进行关联，要求同时能关联上
select * from
(select userId from table_a where dt=20160731) a
left outer join (select userId from table_b where dt=20160731) b on =
left outer join (select userId from table_c where dt=20160731) c on = and =