MySQL中将多个select语句的查询结果合并一起的方法

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1. 背景

* 全并查询结果是将多个 select 语句的查询结果合并到一起。

* 参与合并的结果集需要字段统一。

* 字段可以用空字符串''代替。

2. 合并查询结果实战 [ users1 and users2 ]

* 查看 users1 表和 users2 表结构

mysql> desc users1;+-------+---------------+------+-----+---------+----------------+| Field | Type          | Null | Key | Default | Extra          |+-------+---------------+------+-----+---------+----------------+| id    | bigint(20)    | NO   | PRI | NULL    | auto_increment || name  | varchar(64)   | NO   |     | NULL    |                || sex   | enum('M','F') | NO   |     | NULL    |                || age   | int(11)       | NO   |     | NULL    |                |+-------+---------------+------+-----+---------+----------------+4 rows in set (0.00 sec)mysql> desc users2;+-------+---------------+------+-----+---------+----------------+| Field | Type          | Null | Key | Default | Extra          |+-------+---------------+------+-----+---------+----------------+| id    | bigint(20)    | NO   | PRI | NULL    | auto_increment || name  | varchar(64)   | NO   |     | NULL    |                || sex   | enum('M','F') | NO   |     | NULL    |                || age   | int(11)       | NO   |     | NULL    |                |+-------+---------------+------+-----+---------+----------------+4 rows in set (0.01 sec)

* 查看 users1 表和 users2 表数据

users1和users2表中有相同字段 tom

mysql> select * from users1;+----+------+-----+-----+| id | name | sex | age |+----+------+-----+-----+|  1 | tom  | M   |  25 ||  2 | jak  | F   |  42 |+----+------+-----+-----+2 rows in set (0.00 sec)mysql> select * from users2;+----+-------+-----+-----+| id | name  | sex | age |+----+-------+-----+-----+|  1 | tom   | M   |  25 ||  2 | lisea | M   |  42 |+----+-------+-----+-----+2 rows in set (0.00 sec)

* union 合并并去重

mysql> (select * from users1) union (select * from users2);+----+-------+-----+-----+| id | name  | sex | age |+----+-------+-----+-----+|  1 | tom   | M   |  25 ||  2 | jak   | F   |  42 ||  2 | lisea | M   |  42 |+----+-------+-----+-----+3 rows in set (0.00 sec)

* union all 只全并不去重

mysql> (select * from users1) union all (select * from users2);+----+-------+-----+-----+| id | name  | sex | age |+----+-------+-----+-----+|  1 | tom   | M   |  25 ||  2 | jak   | F   |  42 ||  1 | tom   | M   |  25 ||  2 | lisea | M   |  42 |+----+-------+-----+-----+4 rows in set (0.01 sec)

* 查看union 性能分析

[ 使用了临时表 ]

mysql> explain (select * from users1) union (select * from users2);+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+|  1 | PRIMARY      | users1     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL            ||  2 | UNION        | users2     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL            || NULL | UNION RESULT |  | NULL       | ALL  | NULL        | NULL | NULL    | NULL | NULL |     NULL | Using temporary |+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+3 rows in set, 1 warning (0.01 sec)

* 查看union all 性能分析

[ 未使用临时表 ]

mysql> explain (select * from users1) union all (select * from users2);+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+|  1 | PRIMARY     | users1 | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL  ||  2 | UNION       | users2 | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL  |+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+2 rows in set, 1 warning (0.01 sec)

3. union 与 union all 总结

* union 相对于 union all多了一步去重操作，此操作会创建临时表，降低性能。

* 当两边结果集数据相对都确定了唯一性，推荐使用union all。

4. 总结

以需求驱动技术，技术本身没有优略之分，只有业务之分。

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