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 | <union1,2> | 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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时间： 2024-10-21 04:49:33