SQL UNION 操作符

UNION 操作符用于合并两个或多个 SELECT 语句的结果集。

请注意，UNION 内部的每个 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。同时，每个 SELECT 语句中的列的顺序必须相同。

SQL UNION 语法

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;

SQL UNION ALL 语法

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

演示数据库

在本教程中，我们将使用 RUNOOB 样本数据库。

下面是选自 "Websites" 表的数据：

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | 芝麻教程      | http://www.web3.xin/    | 4689  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 7  | stackoverflow | http://stackoverflow.com/ |   0 | IND     |
+----+---------------+---------------------------+-------+---------+

mysql> SELECT * FROM apps;
+----+------------+-------------------------+---------+
| id | app_name   | url                     | country |
+----+------------+-------------------------+---------+
|  1 | QQ APP     | http://im.qq.com/       | CN      |
|  2 | 微博 APP | http://weibo.com/       | CN      |
|  3 | 淘宝 APP | https://www.taobao.com/ | CN      |
+----+------------+-------------------------+---------+
3 rows in set (0.00 sec)

SQL UNION 实例

下面的 SQL 语句从 "Websites" 和 "apps" 表中选取所有不同的country（只有不同的值）：

实例

SELECT country FROM Websites
UNION
SELECT country FROM apps
ORDER BY country; 

mysql> select country from websites
    -> union
    -> select country from apps
    -> order by country;
+---------+
| country |
+---------+
| CN      |
| IND     |
| USA     |
+---------+
3 rows in set (0.00 sec)

SQL UNION ALL 实例

下面的 SQL 语句使用 UNION ALL 从 "Websites" 和 "apps" 表中选取所有的country（也有重复的值）：

实例

SELECT country FROM Websites
UNION ALL
SELECT country FROM apps
ORDER BY country; 

mysql> select country from websites
    -> union all
    -> select country from apps
    -> order by country desc;
+---------+
| country |
+---------+
| USA     |
| USA     |
| USA     |
| IND     |
| CN      |
| CN      |
| CN      |
| CN      |
| CN      |
+---------+

下面的 SQL 语句使用 UNION ALL 从 "Websites" 和 "apps" 表中选取所有的中国(CN)的数据（也有重复的值）：

实例

SELECT country, name FROM Websites
WHERE country='CN'
UNION ALL
SELECT country, app_name FROM apps
WHERE country='CN'
ORDER BY country; 

mysql> SELECT country, name FROM Websites
    -> WHERE country='CN'
    -> UNION ALL
    -> SELECT country, app_name FROM apps
    -> WHERE country='CN'
    -> ORDER BY country;
+---------+----------+
| country | name     |
+---------+----------+
| CN      | 淘宝     |
| CN      | 微博     |
| CN      | QQ APP   |
| CN      | 微博 APP |
| CN      | 淘宝 APP |
+---------+----------+
5 rows in set (0.00 sec)