#子查询-某些情况下，当进行查询的时候，需要的条件是另外一个select语句的结果，这个时候就要用到子查询。用于子查询的关键字主要包括：
in、not in、=、!=、exists、not exists等等。
#从emp表中查询出所有部门在dept表中的所有记录

mysql> select * from dept;
+--------+----------+
| deptno | deptname |
+--------+----------+
| 1 | tech |
| 2 | sale |
| 3 | hr |
| 4 | sl |
+--------+----------+
rows in set (0.01 sec)

mysql> select * from emp;
+-------+------------+------------+---------+--------+------+
| ename | birth | hirdate | sal | deptno | age1 |
+-------+------------+------------+---------+--------+------+
| zzx1 | 2000-01-01 | 2000-01-01 | 2000.00 | 1 | 21 |
| zzx1 | 2002-03-09 | 2009-04-03 | 2001.00 | 3 | 22 |
| ttx2 | 2023-04-10 | 2010-03-04 | 4000.00 | 4 | 23 |
| ssss | 2019-01-01 | 2018-01-01 | 5000.00 | 2 | 24 |
+-------+------------+------------+---------+--------+------+
rows in set (0.00 sec)

mysql> select * from emp where deptno in(select deptno from dept);
+-------+------------+------------+---------+--------+------+
| ename | birth | hirdate | sal | deptno | age1 |
+-------+------------+------------+---------+--------+------+
| zzx1 | 2000-01-01 | 2000-01-01 | 2000.00 | 1 | 21 |
| ssss | 2019-01-01 | 2018-01-01 | 5000.00 | 2 | 24 |
| zzx1 | 2002-03-09 | 2009-04-03 | 2001.00 | 3 | 22 |
| ttx2 | 2023-04-10 | 2010-03-04 | 4000.00 | 4 | 23 |
+-------+------------+------------+---------+--------+------+
rows in set (0.00 sec)

#如果子查询记录数唯一，还可以用=代替in

mysql> select * from emp where deptno=(select deptno from dept limit 1);

+-------+------------+------------+---------+--------+------+

| ename | birth | hirdate | sal | deptno | age1 |

+-------+------------+------------+---------+--------+------+

| zzx1 | 2000-01-01 | 2000-01-01 | 2000.00 | 1 | 21 |

+-------+------------+------------+---------+--------+------+
row in set (0.00 sec)

某些情况下，子查询可转化为表连接

mysql> select * from emp where deptno in(select deptno from dept);

+-------+------------+------------+---------+--------+------+

| ename | birth | hirdate | sal | deptno | age1 |

+-------+------------+------------+---------+--------+------+

| zzx1 | 2000-01-01 | 2000-01-01 | 2000.00 | 1 | 21 |

| ssss | 2019-01-01 | 2018-01-01 | 5000.00 | 2 | 24 |

| zzx1 | 2002-03-09 | 2009-04-03 | 2001.00 | 3 | 22 |

| ttx2 | 2023-04-10 | 2010-03-04 | 4000.00 | 4 | 23 |

+-------+------------+------------+---------+--------+------+
rows in set (0.00 sec)

#转换为表连接后
mysql> select emp.* from emp,dept where emp.deptno=dept.deptno;
+-------+------------+------------+---------+--------+------+
| ename | birth | hirdate | sal | deptno | age1 |
+-------+------------+------------+---------+--------+------+
| zzx1 | 2000-01-01 | 2000-01-01 | 2000.00 | 1 | 21 |
| ssss | 2019-01-01 | 2018-01-01 | 5000.00 | 2 | 24 |
| zzx1 | 2002-03-09 | 2009-04-03 | 2001.00 | 3 | 22 |
| ttx2 | 2023-04-10 | 2010-03-04 | 4000.00 | 4 | 23 |
+-------+------------+------------+---------+--------+------+
rows in set (0.00 sec)

#union和union all的主要区别是union all是把结果集直接合并在一起
union是将union all后的结果在进行一次distinct，去除重复记录后的结果。
mysql> select deptno from emp union all select deptno from dept;
+--------+
| deptno |
+--------+
| 1 |
| 3 |
| 4 |
| 2 |
| 1 |
| 2 |
| 3 |
| 4 |
+--------+
rows in set (0.00 sec)

mysql> select deptno from emp union select deptno from dept;
+--------+
| deptno |
+--------+
| 1 |
| 3 |
| 4 |
| 2 |
+--------+
rows in set (0.00 sec)