mysql - 为什么 UNION ALL 不产生 UNION RESULT？

Question

mysql> explain select * from t1 union all select * from t2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | PRIMARY     | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL  |
|  2 | UNION       | t2    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

mysql> explain select * from t1 union select * from t2;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | t1         | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL            |
|  2 | UNION        | t2         | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL            |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+

如上面的SQL语句所示，我很困惑为什么一个联合都不产生联合结果？

t1：

ID	内容
1	t1_787

t2：

ID	内容
1	t2_944

标签： mysql

Answer 1

@李君，

UNION：仅保留唯一记录 UNION ALL：保留所有记录，包括重复记录