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                在hive中,`union`和`union all`都是對兩個子查詢的結果合并,不過還是有區別的。 * `union`會對兩個子查詢的結果去重合并,Hive v1.2+版本支持; * `union all`不會對子查詢結果去重處理。 ```sql select * from left_tbl lt left semi join right_tbl rt on lt.id=rt.id; +--------+----------+---------+--+ | lt.id | lt.name | lt.age | +--------+----------+---------+--+ | 1 | 張三 | 20 | | 2 | 李四 | 22 | +--------+----------+---------+--+ select * from left_tbl lt where lt.id in(select id from right_tbl); +--------+----------+---------+--+ | lt.id | lt.name | lt.age | +--------+----------+---------+--+ | 1 | 張三 | 20 | | 2 | 李四 | 22 | +--------+----------+---------+--+ -- union 合并后并去重 select * from left_tbl union select * from right_tbl; +---------+-----------+----------+--+ | _u1.id | _u1.name | _u1.age | +---------+-----------+----------+--+ | 1 | 張三 | 20 | | 2 | 李四 | 22 | | 3 | 王五 | 25 | | 4 | 王五 | 25 | +---------+-----------+----------+--+ -- union all 合并后沒有去重 select * from left_tbl union all select * from right_tbl; +---------+-----------+----------+--+ | _u1.id | _u1.name | _u1.age | +---------+-----------+----------+--+ | 1 | 張三 | 20 | | 2 | 李四 | 22 | | 3 | 王五 | 25 | | 1 | 張三 | 20 | | 2 | 李四 | 22 | | 4 | 王五 | 25 | +---------+-----------+----------+--+ ``` <br/> **`union`與`union all`的使用條件(A `union all` B):** (1)A表和B表的列數必須一樣多; (2)B表的列名必須與A表的列名一一對應,如果是 B `union all` A 則反過來; (3)不需要列的數據類型一致;
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