1. 背景
* 全並查詢結果是將多個 select 語句的查詢結果合併到一起。
* 參與合併的結果集需要字段統一。
* 字段可以用空字符串''代替。
2. 合併查詢結果實戰 [ users1 and users2 ]
* 查看 users1 表和 users2 表結構
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 表數據
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 合併並去重
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 只全並不去重
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
[ 使用了臨時表 ]
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
[ 未使用臨時表 ]
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 與 union all 總結
* union 相對於 union all多了一步去重操作，此操作會創建臨時表，降低性能。
* 當兩邊結果集數據相對都確定了唯一性，推薦使用union all。
4. 總結
以需求驅動技術，技術本身沒有優略之分，只有業務之分。