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 | +----+--------+----------+------+--------+
