部门表、员工表、薪水等级表
1.取得每个部门最高薪水人员名称
第一步：取得每个部门最高薪水作为临时表t
select deptno,max(sal) as maxSal from emp group by deptno 第二步：临时表t与emp表连接条件 e.deptno=t.deptno and e.sal=t.maxSal
select e.ename,t.* from emp e join (select deptno,max(sal) as maxSal from emp group by deptno order by deptno) t on e.deptno=t.deptno and e.sal=t.maxSal
2.哪些人的薪水在部门的平均薪水之上
第一步：取得每个部门平均薪水作为临时表t
SELECT DEPTNO, AVG(SAL) FROM emp GROUP BY DEPTNO;第二步：临时表t与emp表连接条件 e.deptno=t.deptno and e.sal>t.avgSal
SELECT t.*,e.ENAME,e.SAL FROM emp e JOIN (SELECT DEPTNO, AVG(SAL) AS avgSal FROM emp GROUP BY DEPTNO) t ON e.DEPTNO=t.DEPTNO AND e.SAL>t.avgSal ORDER BY DEPTNO; 
3.取得部门中（所有人）平均的薪水等级
平均的薪水等级：先求出每个薪水对应的等级，然后找出薪水等级的平均值
平均薪水的等级：先求出每个部门的平均薪水，再根据平均薪水划分等级
SELECT e.ENAME,e.DEPTNO,e.SAL,s.GRADE FROM emp e JOIN salgrade s ON e.SAL BETWEEN s.LOSAL AND s.HISAL ORDER BY DEPTNO;SELECT e.DEPTNO,AVG(s.GRADE) FROM emp e JOIN salgrade s ON e.SAL BETWEEN s.LOSAL AND s.HISAL GROUP BY e.DEPTNO; 
4. 不准用Max函数取得最高薪水（两种做法）
第一种方案：
SELECT ENAME,SAL FROM emp ORDER BY SAL DESC LIMIT 0,1;第二种方案：表的自连接
SELECT SAL FROM emp WHERE SAL NOT IN(SELECT DISTINCT a.SAL FROM emp a JOIN emp b ON a.SAL
5.取得平均薪水最高的部门的部门编号（两种做法）
第一种方案：降序取第一个
第一步：找出每个部门的平均薪水
SELECT DEPTNO,AVG(SAL) AS avgSAL FROM emp GROUP BY DEPTNO;第二步：降序找出第一个
SELECT DEPTNO FROM emp GROUP BY DEPTNO ORDER BY AVG(SAL) DESC LIMIT 1; 
第二种方案： max函数
第一步同上
第二步：max函数找到最高平均薪水
SELECT MAX(t.avgSAL) FROM (SELECT AVG(SAL) AS avgSAL FROM emp GROUP BY DEPTNO) t第三步：查询完部门编号和平均薪水然后根据条件等于最大平均薪水显示
SELECT DEPTNO,AVG(SAL) AS avgsal FROM emp GROUP BY DEPTNO HAVING avgsal=(SELECT MAX(t.avgSAL) FROM (SELECT AVG(SAL) AS avgSAL FROM emp GROUP BY DEPTNO) t); 
取得平均薪水最高的部门的部门名称
第一步： 找出平均薪水最高的部门
SELECT DEPTNO,AVG(SAL) AS avgsal FROM emp GROUP BY DEPTNO ORDER BY avgsal DESC LIMIT 1;第二步：与部门表连接条件部门编号
SELECT d.DNAME,AVG(e.SAL) AS avgsal FROM emp e JOIN dept d ON e.DEPTNO=d.DEPTNO GROUP BY d.DNAME ORDER BY avgsal DESC LIMIT 1;
7.求平均薪水的等级最低的部门的部门名称（注意这里不能使用limit，存在局限性如果等级最低出现多个部门）
第一步：求各个部门平均薪水
SELECT e.DEPTNO,AVG(e.SAL) AS avgsal FROM emp e GROUP BY DEPTNO ORDER BY avgsal ASC;
第二步：结果与部门表连接条件，查询等级结果
SELECT t.*,s.GRADE FROM (SELECT d.DNAME,AVG(e.SAL) AS avgsal FROM emp e JOIN dept d ON e.DEPTNO=d.DEPTNO GROUP BY d.DNAME ORDER BY avgsal ASC) tJOIN salgrade s ON t.avgsal BETWEEN s.LOSAL AND s.HISAL; 
第三步：查询平均薪水最低对应的等级
SELECT GRADE FROM salgrade WHERE (SELECT AVG(e.SAL) AS avgsal FROM emp e GROUP BY DEPTNO ORDER BY avgsal ASC LIMIT 1) BETWEEN LOSAL AND HISAL; //3第四步：第二步与第三步连接条件显示结果
SELECT t.*,s.GRADE FROM (SELECT d.DNAME,AVG(e.SAL) AS avgsal FROM emp e JOIN dept d ON e.DEPTNO=d.DEPTNO GROUP BY d.DNAME ORDER BY avgsal ASC) tJOIN salgrade s ON t.avgsal BETWEEN s.LOSAL AND s.HISAL WHERE s.GRADE=(SELECT GRADE FROM salgrade WHERE (SELECT AVG(e.SAL) AS avgsal FROM emp e GROUP BY DEPTNO ORDER BY avgsal ASC LIMIT 1) BETWEEN LOSAL AND HISAL);
8.取得比普通员工（员工代码没有在mgr字段上出现）的最高薪水还要高的领导人姓名
第一步：找出普通员工的最高薪水
SELECT MAX(SAL) FROM emp WHERE EMPNO NOT IN (SELECT DISTINCT MGR FROM emp WHERE MGR IS NOT NULL); 
第二步：查询工资比最高薪水高的领导人姓名
SELECT ENAME,SAL FROM emp WHERE SAL>(SELECT MAX(SAL) FROM emp WHERE EMPNO NOT IN (SELECT DISTINCT MGR FROM emp WHERE MGR IS NOT NULL)); 
9.取出薪水最高的前5名员工（这里不需要考虑第6名与第5名工资一样的情况）
SELECT ENAME,SAL FROM emp ORDER BY SAL DESC LIMIT 5;
10.取出薪水最高的第六到第十名的员工
SELECT ENAME,SAL FROM emp ORDER BY SAL DESC LIMIT 5,5;
11.取得最后入职的5名员工
SELECT ENAME,HIREDATE FROM emp ORDER BY HIREDATE DESC LIMIT 5;
12.取得每个薪水等级有多少员工
第一步：取出每个员工薪水对应的等级
SELECT e.ENAME,e.SAL,s.GRADE FROM emp e JOIN salgrade s ON e.SAL BETWEEN s.LOSAL AND s.HISAL;
第二步：根据其等级进行分组并计数
SELECT s.GRADE,COUNT(*) FROM emp e JOIN salgrade s ON e.SAL BETWEEN s.LOSAL AND s.HISAL GROUP BY s.GRADE;
13.列出所有员工及其领导的名字
SELECT a.ENAME '员工',b.ENAME '领导' FROM emp a LEFT JOIN emp b ON a.MGR=b.EMPNO;
14.列出受雇日期早于其直接上级的所有员工的编号，姓名，部门名称
SELECT a.EMPNO '员工号',a.ENAME '员工',a.HIREDATE,b.ENAME '领导',b.HIREDATE,d.DNAME '部门名称' FROM emp a JOIN emp b ON a.MGR=b.EMPNO JOIN dept d ON a.DEPTNO=d.DEPTNO WHERE a.HIREDATE
15.列出部门名称和这些部门的员工信息，同时列出那些没有员工的部门（右外连接）
SELECT e.*,d.DNAME FROM emp e RIGHT JOIN dept d ON e.DEPTNO=d.DEPTNO;
16.列出至少有5个员工的所有部门
SELECT e.DEPTNO,d.DNAME FROM emp e JOIN dept d ON e.DEPTNO=d.DEPTNO GROUP BY DEPTNO HAVING COUNT(*)>=5; 
17.列出薪资比“Smith”高的所有员工信息
SELECT ENAME,SAL FROM emp WHERE SAL >(SELECT SAL FROM emp WHERE ENAME='SMITH');
18.列出所有（“CLERK”）办事员的姓名及其部门名称，部门的人数
第一步：列出所有（“CLERK”）办事员的姓名及其部门名称 临时表t1
SELECT e.ENAME,e.JOB,d.DNAME,d.DEPTNO FROM emp e JOIN dept d ON e.DEPTNO=d.DEPTNO WHERE e.JOB='CLERK';
第二步：列出部门的人数 临时表t2
SELECT DEPTNO,COUNT(*) FROM emp GROUP BY DEPTNO;
第三步：两张临时表连接
SELECT t1.*,t2.deptCount FROM (SELECT e.ENAME,e.JOB,d.DNAME,d.DEPTNO FROM emp e JOIN dept d ON e.DEPTNO=d.DEPTNO WHERE e.JOB='CLERK') t1 JOIN (SELECT DEPTNO,COUNT(*) AS deptCount FROM emp GROUP BY DEPTNO) t2 ON t1.DEPTNO=t2.DEPTNO;
来源地址：https://blog.csdn.net/bubbleJessica/article/details/129432250
