【Mysql教程】数据库里如何联合查询 union方法详解

所需工具：

Mysql

聪明的大脑

勤劳的双手

注意：本站只提供教程，不提供任何成品+工具+软件链接，仅限用于学习和研究，禁止商业用途，未经允许禁止转载/分享等

教程如下

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

 	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

 	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：distinct
 	select * from my_student
 	union
 	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 |
 	+----+--------+----------+------+--------+
 	 
 	 
 	select * from my_student
 	union all
 	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 |
 	| 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_student
 	union all
 	select 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

 	(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 |
 	+----+--------+----------+------+--------+