一、子查询
子查询就是位于SELECT、UPDATE、或DELETE语句中内部的查询
二、子查询的分类
单行子查询
返回零行或一行
多行子查询
返回一行或多行
多列子查询
返回多列
相关子查询
引用外部SQL语句中的一列或多列
嵌套子查询
位于其它子查询中的查询
三、子查询语法
SELECT select_list
FROM table
WHERE expr operator
(SELECT select_list
FROM table);
子查询(内部查询)在执行主查询之前执行一次,然后主查询(外部查询)会使用该子查询的结果
四、子查询的规则
将子查询括在括号中 将子查询放置在比较条件的右侧 只有在执行排序Top-N分析时，子查询中才需要使用ORDER BY 子句 单行运算符用于单行子查询，多行运算符用于多行子查询
五、单行子查询
仅返回一行
使用单行的表较运算符：= ,>, >= ,< , <= ,<>
在WHERE 子句中使用子查询
SQL> select ename,job from emp
where empno = (
select empno from emp
where mgr = 7902 );
ENAME JOB
---------- ---------
SMITH CLERK
--使用分组函数的子查询
SQL> select ename,job,sal
from emp
where sal >
(select avg(sal) from emp);
ENAME JOB SAL
---------- --------- ----------
JONES MANAGER 2975
BLAKE MANAGER 2850
CLARK MANAGER 2450
SCOTT ANALYST 3000
KING PRESIDENT 5000
FORD ANALYST 3000
--在HAVING子句中使用子查询
SQL> select deptno,min(sal)
2 from emp
3 group by deptno
4 having min(sal) >
5 (select min(sal)
6 from emp
7 where deptno = 20);
DEPTNO MIN(SAL)
---------- ----------
30 950
10 1300
--在FROM 子句中使用子查询
SQL> select empno,ename
2 from
3 (select empno,ename
4 from emp
5 where deptno = 20);
EMPNO ENAME
---------- ----------
7369 SMITH
7566 JONES
7788 SCOTT
7876 ADAMS
7902 FORD
--单行子查询中的常见错误
--子查询的结果返回多于一行
SQL> select empno,ename
2 from emp
3 where sal =
4 (select sal
5 from emp
6 where deptno = 20);
(select sal
*
ERROR at line 4:
ORA-01427: single-row subquery returns more than one row
--子查询中不能包含ORDER BY子句
SQL> select empno,ename
2 from emp
3 where sal >
4 (select avg(sal)
5 from emp
6 order by empno);
order by empno)
*
ERROR at line 6:
ORA-00907: missing right parenthesis
--子查询内部没有返回行，如下语句可以正确执行，但没有数据返回
SQL> select ename,job
2 from emp
3 where empno =
4 (select empno
5 from emp
6 where mgr = 8000);
no rows selected
六、多行子查询
返回多个行
使用多行比较运算符IN ,ANY ,ALL
在多行子查询中使用IN 操作符 */
idle> select empno,ename,job
from emp
where sal in
(select max(sal)
from emp
group by deptno);
2 3 4 5 6
EMPNO ENAME JOB
---------- ---------- ---------
7698 BLAKE MANAGER
7788 SCOTT ANALYST
7839 KING PRESIDENT
7902 FORD ANALYST
--在多行子查询中使用ANY 操作符
idle> select empno,ename,job
from emp
where sal < any
(select avg(sal)
from emp
group by deptno);
2 3 4 5 6
EMPNO ENAME JOB
---------- ---------- ---------
7369 SMITH CLERK
7900 JAMES CLERK
7876 ADAMS CLERK
7521 WARD SALESMAN
7654 MARTIN SALESMAN
7934 MILLER CLERK
7844 TURNER SALESMAN
7499 ALLEN SALESMAN
7782 CLARK MANAGER
7698 BLAKE MANAGER
10 rows selected.
--在多行子查询中使用ALL 操作符
idle> select empno,ename,job
from emp
where sal > all
(select avg(sal)
from emp
group by deptno);
2 3 4 5 6
EMPNO ENAME JOB
---------- ---------- ---------
7566 JONES MANAGER
7788 SCOTT ANALYST
7902 FORD ANALYST
7839 KING PRESIDENT
/*
七、相关子查询
子查询中使用了主查询中的某些字段，主查询每扫描一行都要执行一次子查询 */
--查询工资高于同一部门的员工的部门号，姓名，工资
idle> select deptno,ename,sal
from emp outer
where sal >
(select avg(sal)
from emp inner
where inner.deptno = outer.deptno);
2 3 4 5 6
DEPTNO ENAME SAL
---------- ---------- ----------
30 ALLEN 1600
20 JONES 2975
30 BLAKE 2850
20 SCOTT 3000
10 KING 5000
20 FORD 3000
6 rows selected.
--查询负责管理其它员工的员工记录(使用exists)
idle> select empno,ename
from emp outer
where exists
(select empno
from emp inner
where inner.mgr = outer.empno);
2 3 4 5 6
EMPNO ENAME
---------- ----------
7566 JONES
7698 BLAKE
7782 CLARK
7788 SCOTT
7839 KING
7902 FORD
6 rows selected.
--查询不管理其它员工的职员(not exists)
idle> select empno,ename
from emp outer
where not exists
(select empno
from emp inner
where inner.mgr = outer.empno); 2 3 4 5 6
EMPNO ENAME
---------- ----------
7369 SMITH
7499 ALLEN
7521 WARD
7654 MARTIN
7844 TURNER
7876 ADAMS
7900 JAMES
7934 MILLER
8 rows selected.
EXISTS 和NOT EXISTS 与IN 和NOT IN 的比较
EXISTS与IN的不同：
EXISTS只检查行的存在性,IN 要检查实际值的存在性(一般情况下EXISTS的性能高于IN)
NOT EXISTS 和NOT IN
当值列表中包含空值的情况下,NOT EXISTS 则返回true,而NOT IN 则返回false.
--看下面的查询，查询部门号不在emp表中出现的部门名称及位置
idle> select deptno,dname,loc
from dept d
where not exists
(select 1
from emp e
where e.deptno = d.deptno);
2 3 4 5 6
DEPTNO DNAME LOC
---------- -------------- -------------
40 OPERATIONS BOSTON
--IN与空值
idle> SELECT * FROM emp e WHERE e.empno NOT IN ( SELECT 7369 FROM dual UNION ALL SELECT NULL FROM dual ); 2 3 4 5 6 7 no rows selected idle> idle> SELECT * FROM emp e WHERE e.empno IN (‘7369‘,NULL); 2 3 EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ---------- --------- ---------- ---------- ---------- ---------- ---------- 7369 SMITH CLERK 7902 1980-12-17 800 20
注：子查询要包含在括号内
子查询一般放在比较条件的右侧
除非进行TOP－N 分析，否则不要在子查询中使用ORDER BY。 */
/*
八、多列子查询
1、成对比较
查询工资为部门最高的记录
idle> select * from scott.emp
where (sal,job) in
(select max(sal),job from scott.emp group by job);
2 3
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ---------- ---------- ---------- ----------
7934 MILLER CLERK 7782 1982-01-23 1300 10
7499 ALLEN SALESMAN 7698 1981-02-20 1600 300 30
7839 KING PRESIDENT 1981-11-17 5000 10
7566 JONES MANAGER 7839 1981-04-02 2975 20
7902 FORD ANALYST 7566 1981-12-03 3000 20
7788 SCOTT ANALYST 7566 1987-04-19 3000 20
6 rows selected.
2、非成对比较,实现了与上述类似的功能
idle> select * from scott.emp
where sal in (select max(sal) from scott.emp group by job)
and job in (select distinct job from scott.emp);
2 3
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ---------- ---------- ---------- ----------
7934 MILLER CLERK 7782 1982-01-23 1300 10
7499 ALLEN SALESMAN 7698 1981-02-20 1600 300 30
7566 JONES MANAGER 7839 1981-04-02 2975 20
7788 SCOTT ANALYST 7566 1987-04-19 3000 20
7902 FORD ANALYST 7566 1981-12-03 3000 20
7839 KING PRESIDENT 1981-11-17 5000 10
九、嵌套子查询
即位于子查询内部的子查询，嵌套层数最多可达层。然而应尽量避免使用嵌套子查询，使用表连接的查询性能会更高
idle> select deptno,Num_emp
from (select deptno,count(empno) as Num_emp from emp group by deptno) d
where Num_emp > 3;
2 3
DEPTNO NUM_EMP
---------- ----------
30 6
20 5
注意：子查询对空值的处理
除了count(*)外，都会忽略掉空值
