联合查询
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 |
+----+--------+----------+------+--------+
