这里主要分享一下联合查询的用法。
MySQL使用UNION和UNION ALL实现数据的联合查询。
假设有以下两个表：
mysql> select * from staff; +----+----------+-------+ | id | name | slary | +----+----------+-------+ | 1 | guoding | 3200 | | 2 | dingtao | 2800 | | 3 | haofugui | 3500 | | 4 | guoming | 4000 | | 5 | haotian | 2900 | | 6 | fengfei | 3200 | | 7 | guoting | 2600 | +----+----------+-------+ 7 rows in set (0.00 sec) mysql> select * from staff_1; +----+----------+-------+ | id | name | slary | +----+----------+-------+ | 1 | guoding | 3200 | | 2 | liding | 2700 | | 3 | haofugui | 3500 | | 4 | xiaoli | 3600 | | 5 | yazhi | 3200 | | 6 | yuanfei | 3200 | | 7 | guoting | 3500 | +----+----------+-------+ 7 rows in set (0.00 sec)
mysql> select * from staff_2;
+----+-------+----------+
| id | slary | name |
+----+-------+----------+
| 1 | 3200 | guoding |
| 2 | 2700 | liding |
| 3 | 3500 | haofugui |
| 4 | 3600 | xiaoli |
| 5 | 3200 | yazhi |
| 6 | 3200 | yuanfei |
| 7 | 3500 | guoting |
+----+-------+----------+
7 rows in set (0.00 sec)
mysql> select * from staff_3;
+----------+-------+
| name | slary |
+----------+-------+
| guoding | 3200 |
| liding | 2700 |
| haofugui | 3500 |
| xiaoli | 3600 |
+----------+-------+
4 rows in set (0.00 sec)
1. UNION
语句：SELECT column_name FROM table1 UNION SELECT column_name FROM table2
说明：1）用于合并两个或多个 SELECT 语句的结果集，并消去表中任何重复行；
2）UNION 内部的 SELECT 语句必须拥有相同数量的列，列也必须拥有相似的数据类型；
3）同时，每条 SELECT 语句中的列的顺序必须相同；
4）如果子句中有order by,limit等，需用括号()包起来，推荐放到所有子句之后，即对最终合并的结果来排序或筛选。
示例：
mysql> select * from staff union select * from staff_1;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| 1 | guoding | 3200 |
| 2 | dingtao | 2800 |
| 3 | haofugui | 3500 |
| 4 | guoming | 4000 |
| 5 | haotian | 2900 |
| 6 | fengfei | 3200 |
| 7 | guoting | 2600 |
| 2 | liding | 2700 |
| 4 | xiaoli | 3600 |
| 5 | yazhi | 3200 |
| 6 | yuanfei | 3200 |
| 7 | guoting | 3500 |
+----+----------+-------+
12 rows in set (0.00 sec)
mysql> select * from staff union select * from staff_2;
+----+----------------------+----------------------+
| id | name | slary |
+----+----------------------+----------------------+
| 1 | guoding | 3200 |
| 2 | dingtao | 2800 |
| 3 | haofugui | 3500 |
| 4 | guoming | 4000 |
| 5 | haotian | 2900 |
| 6 | fengfei | 3200 |
| 7 | guoting | 2600 |
| 1 | 3200 | guoding |
| 2 | 2700 | liding |
| 3 | 3500 | haofugui |
| 4 | 3600 | xiaoli |
| 5 | 3200 | yazhi |
| 6 | 3200 | yuanfei |
| 7 | 3500 | guoting |
+----+----------------------+----------------------+
14 rows in set (0.00 sec)
mysql> select * from staff union select name from staff_1;
ERROR 1222 (21000): The used SELECT statements have a different number of columns
mysql> select * from staff union select * from staff_3;
ERROR 1222 (21000): The used SELECT statements have a different number of columns
mysql> (select * from staff order by id asc) union (select * from staff_1 order by slary desc);
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| 1 | guoding | 3200 |
| 2 | dingtao | 2800 |
| 3 | haofugui | 3500 |
| 4 | guoming | 4000 |
| 5 | haotian | 2900 |
| 6 | fengfei | 3200 |
| 7 | guoting | 2600 |
| 2 | liding | 2700 |
| 4 | xiaoli | 3600 |
| 5 | yazhi | 3200 |
| 6 | yuanfei | 3200 |
| 7 | guoting | 3500 |
+----+----------+-------+
12 rows in set (0.00 sec)
2. UNION ALL
作用及规则与UNION相同，区别是，UNION ALL不消除重复行
示例：
mysql> select * from staff union select * from staff_1; +----+----------+-------+ | id | name | slary | +----+----------+-------+ | 1 | guoding | 3200 | | 2 | dingtao | 2800 | | 3 | haofugui | 3500 | | 4 | guoming | 4000 | | 5 | haotian | 2900 | | 6 | fengfei | 3200 | | 7 | guoting | 2600 | | 2 | liding | 2700 | | 4 | xiaoli | 3600 | | 5 | yazhi | 3200 | | 6 | yuanfei | 3200 | | 7 | guoting | 3500 | +----+----------+-------+ 12 rows in set (0.00 sec) mysql> select * from staff union all select * from staff_1; +----+----------+-------+ | id | name | slary | +----+----------+-------+ | 1 | guoding | 3200 | | 2 | dingtao | 2800 | | 3 | haofugui | 3500 | | 4 | guoming | 4000 | | 5 | haotian | 2900 | | 6 | fengfei | 3200 | | 7 | guoting | 2600 | | 1 | guoding | 3200 | | 2 | liding | 2700 | | 3 | haofugui | 3500 | | 4 | xiaoli | 3600 | | 5 | yazhi | 3200 | | 6 | yuanfei | 3200 | | 7 | guoting | 3500 | +----+----------+-------+ 14 rows in set (0.00 sec)
