sql第5天 子查询

子查询

语法：

SELECT A FROM B WHERE C GROUP  BY D HAVING E ORDER  BY F;

• 单行单列

SELECT EMPNO FROM EMP WHERE SAL =  800;

→

• 单行多列

SELECT EMPNO,ENAME,JOB FROM EMP WHERE SAL= 800;

→

• 多行单列

SELECT EMPNO FROM EMP;

→

• 多行多列

SELECT EMPNO,ENAME,JOB FROM EMP;

→

SELECT 子查询

总结：

• 多列不能作为SELECT子查询
• 多行不能作为SELECT子查询

FROM子查询

总结：

• FROM 子查询后面可以跟所有的子查询

WHERE 子查询

应用：

查询 SMITH所在部门的部门平均薪资
→

SELECT AVG(SAL) FROM EMP WHERE DEPTNO = (SELECT DEPTNO FROM EMP WHERE ENAME='SMITH');

多行多列的WHERE子查询：

SELECT * FROM EMP WHERE (EMPNO,ENAME,SAL) IN (SELECT EMPNO,ENAME,SAL FROM EMP);--这里的括号都不能省略不写昂~

总结：

• WHERE 子查询全适用，只是适用于不同的条件

GROUP BY子查询

总结：
该子查询不可用

HAVING 子查询

总结：
HAVING 子查询全适用，只是适用于不同的条件

ORDER BY 子查询

总结：
ORDER BY 子查询只适用于单行单列子查询

注意:若子查询中含有函数，需得给函数列起个别名，才能在外层查询语句中被引用

应用：
查询部门平均薪资
→

SELECT AVG(SAL) FROM EMP GROUP BY DEPTNO;

再对部门平均薪资查询其平均值
→

SELECT AVG(AVG(SAL)) FROM (SELECT AVG(SAL) FROM EMP GROUP BY DEPTNO);--报错,SAL无效字符

因为AVG(AVG(SAL))中的SAL在表SELECT AVG(SAL) FROM EMP GROUP BY DEPTNO中不存在，所以我们可以给它起个别名，变成
→

SELECT AVG(AVG(SAL)) FROM (SELECT AVG(SAL) "SAL" FROM EMP GROUP BY DEPTNO);

总结：什么时候可能会用到子查询呢？

1.当一步不能查询出准确数据时
2.当给定的条件不明确且是随某列变化时
3.当题目的条件是一列随着另一列的值变化时

相关子查询：

子查询语句的执行受主查询语句的影响，子查询语句不能独立运行

不相关子查询：

IN+子查询：

符合子查询语句中结果，即返回查询结果

EXISTS+子查询：

一行一行地，当子查询语句有结果返回时，执行主查询语句

NOT EXISTS+子查询：

一行一行地，当子查询语句返回空值时，主查询语句返回空值

语法：

SELECT COLUMN1,COLUMN2... FROM TABLE_NAME WHERE [NOT] EXISTS(查询语句)

应用：

• 相关非相关的相互转化

SELECT * FROM EMP A 
WHERE EXISTS (
SELECT DEPTNO FROM EMP 
WHERE SAL>2000 
AND DEPTNO<>10 
AND A.DEPTNO = DEPTNO--这一行非常重要
);

←
查询工资大于2000并且工作部门不为20号的人员的全部信息

注意这里不是直接执行SELECT * FROM EMP A而是一行一行地执行SELECT * FROM EMP A
所以最后的结果不是单纯的SELECT * FROM EMP A的结果，
而是过滤出了SELECT * FROM EMP A中
AND DEPTNO<>10 AND A.DEPTNO = DEPTNO的结果，所以最后的结果是10条
而不是SELECT * FROM EMP的14条

• 想要用EXISTS语句实现过滤需要考虑以下几点：
  1.要明确比较的列
  2.把比较的列引入子查询中
  3.在子查询中将引入的比较列和子查询中列联系起来
  4.将两个条件联系以后，根据EXISTS独有的规则决定该条数据是否返回到结果集中

应用：查询工资全部大于2000的部门的员工信息
→

SELECT * FROM EMP WHERE DEPTNO IN(SELECT DISTINCT DEPTNO FROM EMP WHERE SAL >2000  MINUS SELECT DISTINCT DEPTNO FROM EMP WHERE SAL IS NULL OR SAL <=2000);

↑↓

老师补充答案：

SELECT * FROM EMP WHERE DEPTNO IN(SELECT DEPTNO FROM EMP GROUP BY DEPTNO HAVING MIN(SAL)>2000);

应用：查询在纽约工作的员工的编号，姓名，以及职位
→

SELECT EMPNO,ENAME,JOB FROM EMP WHERE DEPTNO IN(SELECT DEPTNO FROM DEPT WHERE LOC IN('NEW YORK'));

** 应用：查询部门平均工资大于2000的员工的姓名及其职位

→

SELECT ENAME,JOB FROM EMP WHERE DEPTNO IN(SELECT DEPTNO FROM EMP GROUP BY DEPTNO HAVING AVG(SAL)>2000);

习题：
1.查询工资大于10号部门平均工资的员工信息
→

SELECT * FROM EMP WHERE SAL>(SELECT AVG(SAL)  FROM EMP WHERE DEPTNO=10);

2.查询工资比20号部门所有人工资都高的员工信息
→

SELECT * FROM EMP WHERE SAL>(SELECT MAX(SAL) FROM EMP WHERE DEPTNO=20 );

↑↓

• 老师补充答案：

SELECT * FROM EMP WHERE SAL> ALL(SELECT SAL FROM EMP WHERE DEPTNO =20);

3.查询工资最低的5名员工信息
→

SELECT E.*,ROWNUM FROM (SELECT * FROM EMP ORDER BY SAL ASC) "E" WHERE ROWNUM<=5;

4.査询哪个部门的平均薪资高于30号部门平均薪资，显示出其部门编号和人数
→

SELECT DEPTNO,COUNT(ENAME) FROM EMP  GROUP BY DEPTNO HAVING AVG(SAL)>(SELECT AVG(SAL) FROM EMP WHERE DEPTNO IN(30));

5.显示部门内最低工资比20部门最低工资要高的部门的部门编号及部门内的最低工资
→

SELECT DEPTNO,MIN(SAL) FROM EMP GROUP BY DEPTNO HAVING MIN(SAL)>(SELECT MIN(SAL) FROM EMP WHERE DEPTNO IN(20));

6.査询比CLERK岗位所有人的收入都少的员工的编号、姓名、岗位和薪资
→

SELECT EMPNO,ENAME,JOB,SAL FROM EMP WHERE SAL<(SELECT MIN(SAL) FROM EMP WHERE JOB='CLERK');

** 7.査询比三个部门平均工资都高的员工编号，员工姓名，岗位和薪资
→

SELECT EMPNO, ENAME, JOB, SAL
  FROM EMP
 WHERE SAL > ALL ( SELECT AVG(SAL) FROM EMP GROUP BY DEPTNO );

• ↑错因总结：聚合函数不能对子查询聚合，用ALL关键字代替

8.显示部门名称为 RESEARCH的员工的姓名和工资
→

SELECT ENAME,SAL FROM EMP WHERE DEPTNO IN (SELECT DEPTNO FROM DEPT WHERE DNAME='RESEARCH');

9.找出工资高于SMITH的员工姓名，工资
→

SELECT ENAME,SAL FROM EMP WHERE SAL>(SELECT SAL FROM EMP WHERE ENAME='SMITH');

10.求公司最高工资的姓名和其工资

SELECT ENAME,SAL FROM EMP WHERE SAL=(SELECT MAX(SAL) FROM EMP);

11.求每个部门最高工资员工的姓名，工资，部门编号
→

12.查找与 SMITH或 JAMES同部门同岗位的其他员工的详细信息
→

13.查询部门名称中带有宇母E的部门下所有员工信息（使用 EXISTS）
→

SELECT * FROM EMP A WHERE EXISTS (SELECT DEPTNO FROM DEPT WHERE DNAME LIKE '%E%' AND DEPTNO =A.DEPTNO);

14.查询工资小于1000的员工所在的部门的部门名称和工作地点
→

SELECT DNAME,LOC FROM DEPT WHERE DEPTNO IN(SELECT DEPTNO FROM EMP WHERE SAL<1000);

15.查询平均工资不小于2000的部门信息（使用 not exists）
→

SELECT * FROM DEPT WHERE DEPTNO IN (SELECT DEPTNO FROM EMP GROUP BY DEPTNO HAVING AVG(SAL)>=2000);

16.查询出销售部（SALES）下面的员工姓名，工作，工资
→

SELECT ENAME,JOB,SAL FROM EMP WHERE DEPTNO IN(SELECT DEPTNO FROM DEPT WHERE DNAME IN('SALES'));

17.查询出EMP表中比所有销售员（“SALESMAN”）工资低的员工姓名、工作、工资
→

SELECT ENAME,JOB,SAL FROM EMP WHERE SAL<(SELECT MIN(SAL) FROM EMP WHERE JOB IN('SALESMAN'));

*** 18.查询工资降序排在第4到第8名的员工
→
SELECT * FROM (SELECT EMP.,ROWNUM R FROM EMP ORDER BY EMP.SAL) E WHERE E.R BETWEEN 4 AND 8;
↑↓
SELECT E.,ROWNUM FROM (SELECT * FROM EMP ORDER BY SAL DESC) E WHERE ROWNUM<9
MINUS
SELECT E.*,ROWNUM FROM (SELECT * FROM EMP ORDER BY SAL DESC) E WHERE ROWNUM<4;

• 错误原因：什么时候必须要起别名？
  1.当牵扯到rownum和多个表时，大概率要起别名

19.查询没有员工的部门的部门名称和工作地点
→

SELECT DNAME,LOC FROM DEPT WHERE NOT EXISTS(SELECT EMPNO FROM EMP WHERE EMP.DEPTNO=DEPT.DEPTNO);

↑↓

SELECT DNAME,LOC FROM DEPT WHERE DEPTNO NOT IN (SELECT DEPTNO FROM EMP );

20.求入职时间相同的员工
→

SELECT * FROM EMP WHERE HIREDATE IN (SELECT HIREDATE FROM EMP GROUP BY HIREDATE HAVING COUNT(*) >=2 );

21.显示比工资最高的员工参加工作时间晚的员工姓名和参加工作时间
→

SELECT ENAME,HIREDATE FROM EMP WHERE HIREDATE>(SELECT HIREDATE FROM EMP WHERE SAL=(SELECT MAX(SAL) FROM EMP ));

习题：
1.查询部门编号与(工资大于所有以S开头的员工的工资的员工)的部门编号相等的员工信息
→

SELECT * FROM EMP WHERE DEPTNO IN(SELECT DEPTNO FROM EMP WHERE SAL>ALL(SELECT SAL FROM EMP WHERE ENAME LIKE 'S%'));

2.查询和部门平均薪资相等的员工信息
→

SELECT * FROM EMP WHERE (DEPTNO,SAL) IN(SELECT DEPTNO,AVG(SAL) FROM EMP GROUP BY DEPTNO );

3.查询和部门最高薪资相等的员工信息
→

SELECT * FROM EMP WHERE SAL IN(SELECT MAX(SAL) FROM EMP GROUP BY DEPTNO);

4.查询部门中和该部门最高薪资相等的员工信息
→

SELECT * FROM EMP WHERE SAL IN(SELECT MAX(SAL) FROM EMP GROUP BY DEPTNO);

5.查询最高薪资在2000到3000之间的部门的部门信息
→

SELECT * FROM DEPT WHERE DEPTNO IN(SELECT DEPTNO FROM EMP GROUP BY DEPTNO HAVING MAX(SAL) BETWEEN 2000 AND 3000);

• 6.统计各个部门未达到整体平均薪资水平的员工人数(暂不考虑部门全部大于平均薪资)
  →

SELECT DEPTNO, COUNT(EMPNO)
  FROM EMP
 WHERE EMPNO IN
       (SELECT EMPNO FROM EMP WHERE SAL > (SELECT AVG(SAL) FROM EMP))
 GROUP BY DEPTNO;

↑↑总结：WHERE筛选的是针对FROM后表的数据， HAVING是针对 GROUP BY分组之后的数据进行筛选↑↑

↑↓

SELECT DEPTNO,
       SUM(CASE
             WHEN SAL > (SELECT AVG(SAL) FROM EMP) THEN
              '1'
             ELSE
              '0'
           END)
  FROM EMP
 GROUP BY DEPTNO;

7.A:查询薪资最高的员工信息 B:查询薪资最低的员工信息
→

SELECT * FROM EMP WHERE SAL=(SELECT MAX(SAL) FROM EMP);

→

SELECT * FROM EMP WHERE SAL=(SELECT MIN(SAL) FROM EMP);

8.查询工资等级为5的员工的EMPNO,ENAME,SAL,JOB 列信息
→

SELECT EMPNO,ENAME,SAL,JOB FROM EMP WHERE SAL BETWEEN (SELECT LOSAL FROM SALGRADE WHERE GRADE=5) AND (SELECT HISAL FROM SALGRADE WHERE GRADE= 5);

9.查询在DALLAS和NEW YORK工作的员工的基本信息
→

SELECT * FROM EMP WHERE DEPTNO IN(SELECT DEPTNO FROM DEPT WHERE LOC IN('DALLAS','NEW YORK'));

10.查询工资等级为4且不在CHICAGO工作的员工基本信息
→

SELECT *
  FROM EMP
 WHERE SAL IN
       (SELECT SAL
          FROM SALGRADE
         WHERE SAL BETWEEN (SELECT LOSAL FROM SALGRADE WHERE GRADE = 4) AND
               (SELECT HISAL FROM SALGRADE WHERE GRADE = 4)
           AND DEPTNO <> (SELECT DEPTNO FROM DEPT WHERE LOC IN ('CHICAGO')));

11.假设公司成立于1976，查询五年以后入职的员工信息（不能手动加成1981，然后比较）（一年按照365天计算）
→

SELECT * FROM EMP WHERE HIREDATE >(SELECT TO_DATE('1976/01/01','YYYY/MM/DD')+5*365 FROM DUAL );

12.查询比ALLEN入职晚两天的员工信息
→

SELECT * FROM EMP WHERE HIREDATE IN(SELECT HIREDATE+2 FROM EMP WHERE ENAME='ALLEN');

13.查询比ALLEN入职晚两天的员工所在部门的平均薪资以及最高薪资
→

SELECT AVG(SAL), MAX(SAL)
  FROM EMP
 WHERE DEPTNO IN
       (SELECT DEPTNO
          FROM EMP
         WHERE EMPNO IN
               (SELECT EMPNO
                  FROM EMP
                 WHERE HIREDATE =
                       (SELECT HIREDATE + 2 FROM EMP WHERE ENAME IN ('ALLEN'))));

** 14.查询和20号部门的员工人数相等的部门的员工信息
→

SELECT *
  FROM EMP
 WHERE DEPTNO IN
       (SELECT DEPTNO
          FROM EMP
         GROUP BY DEPTNO
        HAVING COUNT(*) = (SELECT COUNT(*) FROM EMP GROUP BY DEPTNO HAVING DEPTNO =20));

15.查询平均薪资大于30号部门的且员工人数不等于4的部门的员工信息
→

SELECT *
  FROM EMP
 WHERE DEPTNO IN
       (SELECT DEPTNO
          FROM EMP
  		  GROUP BY DEPTNO
           HAVING 4 <> COUNT(*)
           AND AVG(SAL)>(SELECT AVG(SAL) FROM EMP WHERE DEPTNO = 30));

16.统计薪资大于30号部门平均薪资的员工人数
→

SELECT COUNT(*) FROM EMP WHERE SAL>(SELECT AVG(SAL) FROM EMP GROUP BY DEPTNO HAVING DEPTNO IN(30));

17.查询工资大于2000的员工所在的部门的员工信息，并且部门的员工人数和20号部门相同
→

SELECT *
  FROM EMP
 WHERE DEPTNO IN (SELECT DEPTNO FROM EMP WHERE SAL > 2000)
   AND DEPTNO IN (SELECT DEPTNO
                    FROM EMP
                   GROUP BY DEPTNO
                  HAVING COUNT(*) IN (SELECT COUNT(*)
                                       FROM EMP
                                      GROUP BY DEPTNO
                                     HAVING DEPTNO IN(20)));

附加：假设EMP表中增加一列：离职时间 其字段名为 LEAVEDATE 数据类型为 DATE
限制：每天各部门入职，离职的员工人数不等

1）查询每天每个部门入职的人数
2）查询每天每个部门离职的人数
3）查询每天每个部门的入职人数与离职人数差，若其差值为正，则标记+ ；若其差值为负，则标记为-