数据库MySQL-union（联合）

create table emp(
       id tinyint unsigned auto_increment primary key,
       name varchar(20) not null,
       skill set('PHP','mysql','java')
 );
 
insert into emp values (null,'李白',1),(null,'杜甫',2),(null,'白居易',4)
insert into emp values (null,'争青小子',3)

语法：select 语句 union [选项] select 语句 union [选项] select 语句

-- 查询stu表中的姓名和emp表中姓名 结果自动合并的重复的记录
mysql> select stuname from stu union select name from emp;

-- 方法一：
mysql> select * from stu where (stuaddress='上海' and stusex='男') or (stuaddress='北京' and stusex='女');
+--------+---------+--------+--------+---------+------------+------+------+
| stuNo  | stuName | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+---------+--------+--------+---------+------------+------+------+
| s25302 | 李文才       | 男       |     31 |       3 | 上海          |   77 |   76 |
| s25303 | 李斯文       | 女      |     22 |       2 | 北京           |   55 |   82 |
+--------+---------+--------+--------+---------+------------+------+------+
2 rows in set (0.00 sec)

-- 方法二：union
mysql> select * from stu where stuaddress='上海' and stusex='男' union select * from stu where stuaddress='北京' and stusex='女';
+--------+---------+--------+--------+---------+------------+------+------+
| stuNo  | stuName | stuSex | stuAge | stuSeat | stuAddress | ch   | math |
+--------+---------+--------+--------+---------+------------+------+------+
| s25302 | 李文才       | 男       |     31 |       3 | 上海          |   77 |   76 |
| s25303 | 李斯文       | 女      |     22 |       2 | 北京           |   55 |   82 |
+--------+---------+--------+--------+---------+------------+------+------+
2 rows in set (0.00 sec)

结论：union可以将一个复杂的条件转成两个简单的条件

mysql> select stuname from stu union all select name from emp;