前言：
将多个查询结果的结果集合并到一起（纵向合并），字段数不变，多个查询结果的记录数合并
1、应用场景
- 同一张表中不同结果合并到一起展示：男生升高升序，女生升高降序
- 数据量较大的表，进行分表操作，将每张表的数据合并起来显示
2、基本语法
|
1
2
3
|
select 语句union [union 选项]select 语句; |
union 选项 和select 选项基本一致
- distinct 去重，默认
- all 保存所有结果
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
|
mysql> select * from my_student;+----+--------+----------+------+--------+| id | name | class_id | age | gender |+----+--------+----------+------+--------+| 1 | 刘备 | 1 | 18 | 2 || 2 | 李四 | 1 | 19 | 1 || 3 | 王五 | 2 | 20 | 2 || 7 | 张飞 | 2 | 21 | 1 || 8 | 关羽 | 1 | 22 | 2 || 9 | 曹操 | 1 | 20 | NULL |+----+--------+----------+------+--------+-- 默认选项：distinctselect * from my_studentunionselect * from my_student;+----+--------+----------+------+--------+| id | name | class_id | age | gender |+----+--------+----------+------+--------+| 1 | 刘备 | 1 | 18 | 2 || 2 | 李四 | 1 | 19 | 1 || 3 | 王五 | 2 | 20 | 2 || 7 | 张飞 | 2 | 21 | 1 || 8 | 关羽 | 1 | 22 | 2 || 9 | 曹操 | 1 | 20 | NULL |+----+--------+----------+------+--------+select * from my_studentunion allselect * from my_student;+----+--------+----------+------+--------+| id | name | class_id | age | gender |+----+--------+----------+------+--------+| 1 | 刘备 | 1 | 18 | 2 || 2 | 李四 | 1 | 19 | 1 || 3 | 王五 | 2 | 20 | 2 || 7 | 张飞 | 2 | 21 | 1 || 8 | 关羽 | 1 | 22 | 2 || 9 | 曹操 | 1 | 20 | NULL || 1 | 刘备 | 1 | 18 | 2 || 2 | 李四 | 1 | 19 | 1 || 3 | 王五 | 2 | 20 | 2 || 7 | 张飞 | 2 | 21 | 1 || 8 | 关羽 | 1 | 22 | 2 || 9 | 曹操 | 1 | 20 | NULL |+----+--------+----------+------+--------+-- 只需要保证字段数量一样，不需要每次拿到的数据类型都一样-- 只保留第一个select的字段名select id, name, age from my_studentunion allselect name, id, age from my_student;+--------+--------+------+| id | name | age |+--------+--------+------+| 1 | 刘备 | 18 || 2 | 李四 | 19 || 3 | 王五 | 20 || 7 | 张飞 | 21 || 8 | 关羽 | 22 || 9 | 曹操 | 20 || 刘备 | 1 | 18 || 李四 | 2 | 19 || 王五 | 3 | 20 || 张飞 | 7 | 21 || 关羽 | 8 | 22 || 曹操 | 9 | 20 |+--------+--------+------+ |
3、order by的使用
联合查询中，使用order by, select语句必须使用括号
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
|
(select * from my_student where gender = 1 order by age desc)union(select * from my_student where gender = 2 order by age asc);+----+--------+----------+------+--------+| id | name | class_id | age | gender |+----+--------+----------+------+--------+| 2 | 李四 | 1 | 19 | 1 || 7 | 张飞 | 2 | 21 | 1 || 1 | 刘备 | 1 | 18 | 2 || 3 | 王五 | 2 | 20 | 2 || 8 | 关羽 | 1 | 22 | 2 |+----+--------+----------+------+--------+-- order by 要生效，必须使用limit 通常大于表的记录数(select * from my_student where gender = 1 order by age desc limit 10)union(select * from my_student where gender = 2 order by age asc limit 10);+----+--------+----------+------+--------+| id | name | class_id | age | gender |+----+--------+----------+------+--------+| 7 | 张飞 | 2 | 21 | 1 || 2 | 李四 | 1 | 19 | 1 || 1 | 刘备 | 1 | 18 | 2 || 3 | 王五 | 2 | 20 | 2 || 8 | 关羽 | 1 | 22 | 2 |+----+--------+----------+------+--------+ |
到此这篇关于MySQL数据库之联合查询 union的文章就介绍到这了,更多相关MySQL联合查询 union内容请搜索服务器之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持服务器之家！
原文链接：https://blog.51cto.com/u_13567403/5347145
