UNION 用于将多个 SELECT 语句的结果合并到结果集中。第一个 SELECT 语句的列名被用作返回结果的列列名。在每个 SELECT 语句的相应位置列出的选定列应该有相同的数据类型。
测试数据和表结构
mysql> desc books;
+-------------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| create_time | datetime | NO | | NULL | |
| is_hot | tinyint(3) unsigned | NO | | 0 | |
+-------------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
mysql> select * from books;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 1 | Head First Java | 2017-12-04 17:19:36 | 0 |
| 2 | Think in Java | 2017-12-02 17:19:36 | 0 |
| 3 | Clean Code | 2017-12-01 17:19:36 | 0 |
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 5 | JavaScript Definitive Guides | 2017-12-06 17:19:36 | 0 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 7 | The C Programming Language | 2017-12-08 17:19:36 | 0 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
+----+---------------------------------------+---------------------+--------+
8 rows in set (0.00 sec)
组合查询
where 和 union 语句在多数情况下可以实现相同的结果集。where 可以实现的语句，一定可以用 union 实现。而 union 可以实现的语句，where 却不一定能做到。因为，union 是可以针对多张表进行结果集的合并。
查询 id = 1 的记录
mysql> select * from books where id = 1;
+----+-----------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+-----------------+---------------------+--------+
| 1 | Head First Java | 2017-12-04 17:19:36 | 0 |
+----+-----------------+---------------------+--------+
1 row in set (0.00 sec)
查询 is_hot = 1 的记录
mysql> select * from books where is_hot = 1;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
+----+---------------------------------------+---------------------+--------+
3 rows in set (0.00 sec)
使用 union 合并两个查询
mysql> select * from books where id = 1 union select * from books where is_hot = 1;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 1 | Head First Java | 2017-12-04 17:19:36 | 0 |
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
+----+---------------------------------------+---------------------+--------+
4 rows in set (0.00 sec)
当然，我们通过 select * from books where id = 1 or is_hot = 1 也可以做到，但是 union 是不限于表的。也就是说，我们可以从多个表中查询结果集出来，然后 merge 到第一个查询语句查询的结果集中（前提是字段数量和类型一致）。
重复项
默认情况下，我们单单使用 union 语句，会自动帮我们去除重复的项。但是，如果我们使用了 union all 语句，数据库不会帮我们自动去除重复项。
mysql> select * from books where id > 5 union select * from books where is_hot = 1;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 7 | The C Programming Language | 2017-12-08 17:19:36 | 0 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
+----+---------------------------------------+---------------------+--------+
4 rows in set (0.00 sec)
mysql> select * from books where id > 5 union all select * from books where is_hot = 1;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 7 | The C Programming Language | 2017-12-08 17:19:36 | 0 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
+----+---------------------------------------+---------------------+--------+
6 rows in set (0.00 sec)
关于排序
union 中的排序是比较坑的，即使我们在部分结果集这种写了 order by 语句。因为，union 会在合并结果集之后自动的进行排序。意味着在子结果集中的排序，排了也是白排。例如：
mysql> (select * from books where is_hot = 1 order by create_time desc)
-> union
-> (select * from books where is_hot = 0 order by create_time desc);
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
| 1 | Head First Java | 2017-12-04 17:19:36 | 0 |
| 2 | Think in Java | 2017-12-02 17:19:36 | 0 |
| 3 | Clean Code | 2017-12-01 17:19:36 | 0 |
| 5 | JavaScript Definitive Guides | 2017-12-06 17:19:36 | 0 |
| 7 | The C Programming Language | 2017-12-08 17:19:36 | 0 |
+----+---------------------------------------+---------------------+--------+
8 rows in set (0.00 sec)
从查询的意图来看，我们是想让 is_hot = 1 和 is_hot = 0 的结果集各自按 create_time 字段进行排序。但是结果出乎意外，并没有按我们设定的顺序进行。
其实，子结果集是进行了排序的，只不过 union 在合并结果集之后，会自动的对整个结果集进行排序。
假如我们现在有一个需求，is_hot = 1 的是热门书单，is_hot = 0 的是一般书单，然后按热门书单按 create_time 排序，普通书单也按 create_time 排序，返回一个集合。或者分页查询。我们该如何实现这个需求呢？查询语句如下：
mysql> (select * from books where is_hot = 1 order by create_time desc limit 7)
-> union
-> (select * from books where is_hot = 0 order by create_time desc limit 7);
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 7 | The C Programming Language | 2017-12-08 17:19:36 | 0 |
| 5 | JavaScript Definitive Guides | 2017-12-06 17:19:36 | 0 |
| 1 | Head First Java | 2017-12-04 17:19:36 | 0 |
| 2 | Think in Java | 2017-12-02 17:19:36 | 0 |
| 3 | Clean Code | 2017-12-01 17:19:36 | 0 |
+----+---------------------------------------+---------------------+--------+
8 rows in set (0.00 sec)
在每一个查询语句的后面都写上 limit 7 (总条数)。那么分页呢？
mysql> (select id, name, is_hot, create_time from books where is_hot = 1 order by create_time desc limit 7)
-> union
-> (select id, name, is_hot, create_time from books where is_hot = 0 order by create_time desc limit 7)
-> limit 3 offset 0;
+----+---------------------------------------+--------+---------------------+
| id | name | is_hot | create_time |
+----+---------------------------------------+--------+---------------------+
| 8 | Pro Spring Boot | 1 | 2017-12-09 17:19:36 |
| 6 | Java 9 Data Structures and Algorithms | 1 | 2017-12-07 17:19:36 |
| 4 | The Ruby Programming | 1 | 2017-12-05 17:19:36 |
+----+---------------------------------------+--------+---------------------+
3 rows in set (0.00 sec)
mysql> (select id, name, is_hot, create_time from books where is_hot = 1 order by create_time desc limit 7)
-> union
-> (select id, name, is_hot, create_time from books where is_hot = 0 order by create_time desc limit 7)
-> limit 3 offset 3;
+----+------------------------------+--------+---------------------+
| id | name | is_hot | create_time |
+----+------------------------------+--------+---------------------+
| 7 | The C Programming Language | 0 | 2017-12-08 17:19:36 |
| 5 | JavaScript Definitive Guides | 0 | 2017-12-06 17:19:36 |
| 1 | Head First Java | 0 | 2017-12-04 17:19:36 |
+----+------------------------------+--------+---------------------+
3 rows in set (0.00 sec)
多表查询
创建一张杂志表 magazines 并插入两条数据。
mysql> desc magazines;
+-------------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(255) | NO | | NULL | |
| create_time | datetime | NO | | NULL | |
| is_hot | tinyint(2) unsigned | NO | | 0 | |
+-------------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
mysql> select * from magazines;
+----+--------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+--------------+---------------------+--------+
| 1 | 男人装 | 2017-12-05 09:28:06 | 0 |
| 2 | 程序员 | 2017-12-01 09:28:06 | 1 |
+----+--------------+---------------------+--------+
3 rows in set (0.00 sec)
假如需求是从两张表或者多张表中查询出结果集，并且分页显示。那么这时候 where 很显然做不到，但是 union 却可以轻松做到。例如：查询 books 和 magazines 表中 is_hot = 1 的数据。
mysql> select * from books where is_hot = 1
-> union
-> select * from magazines where is_hot = 1;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
| 2 | 程序员 | 2017-12-01 09:28:06 | 1 |
+----+---------------------------------------+---------------------+--------+
4 rows in set (0.01 sec)
区分多表
联合多张表查询的时候，有时候会需要做到区分数据来自于哪一张表。比如，查询热门杂志和热门书籍，并且分别显示来自于哪一张表。
mysql> select *, 'books' as table_name from books where is_hot = 1
-> union
-> select *, 'magazines' as table_name from magazines where is_hot = 1;
+----+---------------------------------------+---------------------+--------+------------+
| id | name | create_time | is_hot | table_name |
+----+---------------------------------------+---------------------+--------+------------+
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 | books |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 | books |
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 | books |
| 2 | 程序员 | 2017-12-01 09:28:06 | 1 | magazines |
+----+---------------------------------------+---------------------+--------+------------+
4 rows in set (0.00 sec)
这样查询出来的结果集，封装成对象集合返回给前端，也方便前端对 books 或者 magazines 打不同的 tag。
具体应用场景
比如我们要对 books 表中的数据进行分页显示，要求热门书籍在上面，普通书籍在下面。热门书籍按 create_time 倒序，普通书籍也按 create_time 倒序。然后分页，每页显示 5 条数据。
第一页的数据：
mysql> (select * from books where is_hot = 1 order by create_time desc limit 8)
-> union
-> (select * from books where is_hot = 0 order by create_time desc limit 8)
-> limit 5 offset 0;
+----+---------------------------------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+---------------------------------------+---------------------+--------+
| 8 | Pro Spring Boot | 2017-12-09 17:19:36 | 1 |
| 6 | Java 9 Data Structures and Algorithms | 2017-12-07 17:19:36 | 1 |
| 4 | The Ruby Programming | 2017-12-05 17:19:36 | 1 |
| 7 | The C Programming Language | 2017-12-08 17:19:36 | 0 |
| 5 | JavaScript Definitive Guides | 2017-12-06 17:19:36 | 0 |
+----+---------------------------------------+---------------------+--------+
5 rows in set (0.01 sec)
第二页的数据：
mysql> (select * from books where is_hot = 1 order by create_time desc limit 8)
-> union
-> (select * from books where is_hot = 0 order by create_time desc limit 8)
-> limit 5 offset 5;
+----+-----------------+---------------------+--------+
| id | name | create_time | is_hot |
+----+-----------------+---------------------+--------+
| 1 | Head First Java | 2017-12-04 17:19:36 | 0 |
| 2 | Think in Java | 2017-12-02 17:19:36 | 0 |
| 3 | Clean Code | 2017-12-01 17:19:36 | 0 |
+----+-----------------+---------------------+--------+
3 rows in set (0.00 sec)
参考资料
https://dev.mysql.com/doc/refman/5.7/en/any-in-some-subqueries.html
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