1. =、in

在主查询执行之前，子查询(内查询)会执行一次，子查询结果被用于主查询(外查询)。

SQL> select ename from emp 
    where sal=(select max(sal) from emp);
SQL> select ename from emp 
    where sal in (select sal from emp where sal>2999);
SQL> select ename from emp where empno in ( select mgr from emp);#担任领导的员工
SQL> select ename from emp where empno not  in ( select mgr from emp);
no rows selected # 这里为空，因为子查询结果存在空值
SQL> select ename,mgr,empno from emp where mgr is null;
ENAME         MGR      EMPNO
---------- ---------- ----------
KING                7839
SQL> select ename from emp  # 不担任领导的员工 
    where empno not  in ( select mgr from emp where mgr is not null);

2. any(some)、all

• > any 大于最小值

SQL> select ename,sal from emp where sal>any(2500,3000);
ENAME         SAL
---------- ----------
JONES        2975
BLAKE        2850
SCOTT        3000
KING         5000
FORD         3000

• > all 大于最大值

SQL> select ename,sal from emp where sal>all(2500,3000);
ENAME         SAL
---------- ----------
KING         5000

• < any 小于最大值

SQL> select ename,sal from emp where sal<any(2500,3000);

ENAME         SAL
---------- ----------
SMITH         800
ALLEN        1600
WARD         1250
JONES        2975
MARTIN       1250
BLAKE        2850

• < all 小于最小值

SQL> select ename,sal from emp where sal<all(2500,3000);

ENAME         SAL
---------- ----------
SMITH         800
ALLEN        1600
WARD         1250
MARTIN       1250
CLARK        2450

SQL> select ename,sal from emp where sal<some(2500,3000); # oracle中，some等值于any

3.减少重复循环查询

• 担任领导的员工

#同下，但循环执行多次
SQL> select ename from emp where empno in ( select mgr from emp); 
SQL> select ename from emp e where exists (select 1 from emp where mgr=e.empno);
# 1为查询mgr=e.empno结果返回的标签，查到即返回1,
# 整个语句一次全表扫描完后，返回所有为1的结果数据

• 不担任领导的员工

SQL> select ename from emp where empno not in ( select mgr from emp where mgr is not null);
SQL> select ename from emp e where not exists (select 1 from emp where mgr=e.empno);
SQL> select ename from emp natural join (select distinct mgr empno from emp);

• 最高薪水

SQL> select ename from emp natural join (select max(sal) sal from emp);
SQL> select ename from emp where sal=(select max(sal) from emp);