/*
1、取得每个部门最高薪水的人员名称
第一步：取得每个部门的最高薪水
select deptno,max(sal) as maxsal from emp group by deptno;
+--------+---------+
| deptno | maxsal |
+--------+---------+
| 10 | 5000.00 |
| 20 | 3000.00 |
| 30 | 2850.00 |
+--------+---------+
第二步：将以上结果当作临时表t，t表和emp e表进行连接，条件是t.deptno = e.deptno and t.maxsal = e.sal
select
e.ename,t.*
from
(select deptno,max(sal) as maxsal from emp group by deptno) t
join
emp e
on
t.deptno = e.deptno and t.maxsal = e.sal;
+-------+--------+---------+
| ename | deptno | maxsal |
+-------+--------+---------+
| BLAKE | 30 | 2850.00 |
| SCOTT | 20 | 3000.00 |
| KING | 10 | 5000.00 |
| FORD | 20 | 3000.00 |
+-------+--------+---------+
2、哪些人的薪水在部门的平均薪水之上
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;
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
第二步：将以上查询结果当作t表，t和emp表连接
条件：部门编号相同，并且emp的sal大于t表的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;
+--------+-------------+-------+---------+
| deptno | avgsal | ename | sal |
+--------+-------------+-------+---------+
| 30 | 1566.666667 | ALLEN | 1600.00 |
| 20 | 2175.000000 | JONES | 2975.00 |
| 30 | 1566.666667 | BLAKE | 2850.00 |
| 20 | 2175.000000 | SCOTT | 3000.00 |
| 10 | 2916.666667 | KING | 5000.00 |
| 20 | 2175.000000 | FORD | 3000.00 |
+--------+-------------+-------+---------+
3.取得部门中（所有人的）平均的薪水等级
第一步：
先确定部门中所有人的薪水等级
select
e.ename,e.sal,e.deptno,s.grade
from
emp e
join
salgrade s
on
e.sal between s.losal and s.hisal;
+--------+---------+--------+-------+
| ename | sal | deptno | grade |
+--------+---------+--------+-------+
| SMITH | 800.00 | 20 | 1 |
| ALLEN | 1600.00 | 30 | 3 |
| WARD | 1250.00 | 30 | 2 |
| JONES | 2975.00 | 20 | 4 |
| MARTIN | 1250.00 | 30 | 2 |
| BLAKE | 2850.00 | 30 | 4 |
| CLARK | 2450.00 | 10 | 4 |
| SCOTT | 3000.00 | 20 | 4 |
| KING | 5000.00 | 10 | 5 |
| TURNER | 1500.00 | 30 | 3 |
| ADAMS | 1100.00 | 20 | 1 |
| JAMES | 950.00 | 30 | 1 |
| FORD | 3000.00 | 20 | 4 |
| MILLER | 1300.00 | 10 | 2 |
+--------+---------+--------+-------+
第二步：基于以上的结果，按照deptno分组，求grade的平均值。
select
t.deptno,avg(t.grade)
from
(select e.ename,e.sal,e.deptno,s.grade from emp e join salgrade s on e.sal between s.losal and s.hisal) t
group by
t.deptno;
另外一种方式：不需要将以上结果看作临时表，直接继续分组即可。
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;
+--------+--------------+
| deptno | avg(t.grade) |
+--------+--------------+
| 10 | 3.6667 |
| 20 | 2.8000 |
| 30 | 2.5000 |
+--------+--------------+
4.不准用组函数(Max)，取得最高薪水（给出两种解决方案）
第一种：降序，limit 1
select ename,sal from emp order by sal desc limit 1;
+-------+---------+
| ename | sal |
+-------+---------+
| KING | 5000.00 |
+-------+---------+
第二种：select max(sal) from emp;
第三种：表的自连接
a表
+---------+
| sal |
+---------+
| 800.00 |
| 1600.00 |
| 1250.00 |
| 2975.00 |
| 1250.00 |
| 2850.00 |
| 2450.00 |
| 3000.00 |
| 5000.00 |
| 1500.00 |
| 1100.00 |
| 950.00 |
| 3000.00 |
| 1300.00 |
+---------+
b表
+---------+
| sal |
+---------+
| 800.00 |
| 1600.00 |
| 1250.00 |
| 2975.00 |
| 1250.00 |
| 2850.00 |
| 2450.00 |
| 3000.00 |
| 5000.00 |
| 1500.00 |
| 1100.00 |
| 950.00 |
| 3000.00 |
| 1300.00 |
+---------+
第一步：
select
distinct a.sal
from
emp a
join
emp b
on
a.sal < b.sal;
+---------+
| sal |
+---------+
| 800.00 |
| 1250.00 |
| 1500.00 |
| 1100.00 |
| 950.00 |
| 1300.00 |
| 1600.00 |
| 2850.00 |
| 2450.00 |
| 2975.00 |
| 3000.00 |
+---------+
第二步：
select sal from emp where sal not in (select distinct a.sal from emp a join emp b on a.sal < b.sal);
+---------+
| sal |
+---------+
| 5000.00 |
+---------+
5.取得平均薪水最高的部门编号（至少给出两种解决方案）
第一种：先获取部门的平均薪水，然后降序，limit 1.
select
deptno,avg(sal) avgsal
from
emp
group by
deptno
order by
avgsal desc
limit 1;
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
+--------+-------------+
第二种:max
第一步:找出每个部门的平均薪水
select deptno,avg(sal) avgsal from emp group by deptno;
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
第二步：找出以上结果中avgsal最大的值。
select avg(sal) as avgsal from emp group by deptno;
+-------------+
| avgsal |
+-------------+
| 2916.666667 |
| 2175.000000 |
| 1566.666667 |
+-------------+
select max(t.avgsal) from (select avg(sal) as avgsal from emp group by deptno) t;
+---------------+
| max(t.avgsal) |
+---------------+
| 2916.666667 |
+---------------+
第三步：
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);
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
+--------+-------------+
6.取得平均薪水最高的部门的部门名称
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;
+------------+-------------+
| dname | avgsal |
+------------+-------------+
| ACCOUNTING | 2916.666667 |
+------------+-------------+
7.求平均薪水的等级最低的部门的部门名称。
注意：
平均薪水最低，等级可能是最低。
平均薪水不是最低，但也有可能是等级最低。
因此有可能多个结果。不能按照等级排序limit获取。
第一步：找出每个部门的平均薪水
select avg(sal) as avgsal,deptno from emp group by deptno;
+-------------+--------+
| avgsal | deptno |
+-------------+--------+
| 2916.666667 | 10 |
| 2175.000000 | 20 |
| 1566.666667 | 30 |
+-------------+--------+
第二步：找出每个部门的平均薪水的等级
以上t表和salgrade表连接，条件：
t.avgsal between s.losal and s.hisal
select
t.*,s.grade
from
(select avg(sal) as avgsal,deptno from emp group by deptno) t
join
salgrade s
on
t.avgsal between s.losal and s.hisal;
+-------------+--------+-------+
| avgsal | deptno | grade |
+-------------+--------+-------+
| 1566.666667 | 30 | 3 |
| 2916.666667 | 10 | 4 |
| 2175.000000 | 20 | 4 |
+-------------+--------+-------+
第三步：
抛开之前的，最低等级你怎么找？
平均薪水最低的对应的等级一定是最低的。
select avg(sal) as avgsal from emp group by deptno order by avgsal asc limit 1;
+-------------+
| avgsal |
+-------------+
| 1566.666667 |
+-------------+
select grade from salgrade where (select avg(sal) as avgsal from emp group by deptno order by avgsal asc limit 1) between losal and hisal;
+-------+
| grade |
+-------+
| 3 |
+-------+
第四步：
先将dept表和emp表连接，以dname分组
select
t.*,s.grade
from
(select avg(e.sal) as avgsal,d.dname from emp e join dept d on e.deptno = d.deptno group by d.dname) t
join
salgrade s
on
t.avgsal between s.losal and s.hisal;
+-------------+------------+-------+
| avgsal | dname | grade |
+-------------+------------+-------+
| 1566.666667 | SALES | 3 |
| 2916.666667 | ACCOUNTING | 4 |
| 2175.000000 | RESEARCH | 4 |
+-------------+------------+-------+
然后按照，where 最低等级 = 3
select
t.*,s.grade
from
(select avg(e.sal) as avgsal,d.dname from emp e join dept d on e.deptno = d.deptno group by d.dname) t
join
salgrade s
on
t.avgsal between s.losal and s.hisal
where
s.grade = (select grade from salgrade where (select avg(sal) as avgsal from emp group by deptno order by avgsal asc limit 1) between losal and hisal);
+-------------+-------+-------+
| avgsal | dname | grade |
+-------------+-------+-------+
| 1566.666667 | SALES | 3 |
+-------------+-------+-------+
8.取得比普通员工（员工代码没有在mgr字段上出现的）的最高薪水还要高的领导人姓名。
领导代码
mysql> select distinct mgr from emp;
+------+
| mgr |
+------+
| 7902 |
| 7698 |
| 7839 |
| 7566 |
| NULL |
| 7788 |
| 7782 |
+------
员工编号没有在以上范围内的都是普通员工。
第一步：找到普通员工的最高薪水：
select max(sal) from emp where 普通员工;
select max(sal) from emp where empno not in (select distinct mgr from emp);
+----------+
| max(sal) |
+----------+
| NULL |
+----------+
因为列表中有NULL，与NULL的任何运算结果都是NULL，影响结果。
需要手动排除NULL
not in在使用的时候，后面小括号中记得排除NULL。
select distinct mgr from emp where mgr is not null;
+------+
| mgr |
+------+
| 7902 |
| 7698 |
| 7839 |
| 7566 |
| 7788 |
| 7782 |
+------+
select max(sal) from emp where empno not in (select distinct mgr from emp where mgr is not null);
+----------+
| max(sal) |
+----------+
| 1600.00 |
+----------+
第二步：找出高于1600的
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));
+-------+---------+
| ename | sal |
+-------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
| FORD | 3000.00 |
+-------+---------+
第三步：
比”普通员工的最高薪水“还要高的一定是领导！
没毛病！
因为已经是普通员工的最高薪水了，还要高，一定是不是普通员工，那就是领导了。
9. 取得薪水最高的前五名员工
select ename,sal from emp order by sal desc limit 5;
+-------+---------+
| ename | sal |
+-------+---------+
| KING | 5000.00 |
| SCOTT | 3000.00 |
| FORD | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
+-------+---------+
10.取得薪水最高的第六到第十名员工
select ename,sal from emp order by sal desc limit 5, 5;
+--------+---------+
| ename | sal |
+--------+---------+
| CLARK | 2450.00 |
| ALLEN | 1600.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
| MARTIN | 1250.00 |
+--------+---------+
11.取得最后入职的5名员工
日期也可以降序，升序。
select ename,hiredate from emp order by hiredate desc limit 5;
+--------+------------+
| ename | hiredate |
+--------+------------+
| ADAMS | 1987-05-23 |
| SCOTT | 1987-04-19 |
| MILLER | 1982-01-23 |
| FORD | 1981-12-03 |
| JAMES | 1981-12-03 |
+--------+------------+
12.取得每个薪水等级有多少员工
分组count。
第一步:每个员工的薪水等级
select
e.ename,e.sal,s.grade
from
emp e
join
salgrade s
on
e.sal between s.losal and s.hisal;
+--------+---------+-------+
| ename | sal | grade |
+--------+---------+-------+
| SMITH | 800.00 | 1 |
| ALLEN | 1600.00 | 3 |
| WARD | 1250.00 | 2 |
| JONES | 2975.00 | 4 |
| MARTIN | 1250.00 | 2 |
| BLAKE | 2850.00 | 4 |
| CLARK | 2450.00 | 4 |
| SCOTT | 3000.00 | 4 |
| KING | 5000.00 | 5 |
| TURNER | 1500.00 | 3 |
| ADAMS | 1100.00 | 1 |
| JAMES | 950.00 | 1 |
| FORD | 3000.00 | 4 |
| MILLER | 1300.00 | 2 |
+--------+---------+-------+
第二步：按照grade分组，进行统计
select
s.grade,count(*)
from
emp e
join
salgrade s
on
e.sal between s.losal and s.hisal
group by
s.grade;
+-------+----------+
| grade | count(*) |
+-------+----------+
| 1 | 3 |
| 2 | 3 |
| 3 | 2 |
| 4 | 5 |
| 5 | 1 |
+-------+----------+
13.面试题
14.列出所有员工及领导的名字
注意是外连接
select a.ename '员工',b.ename '领导' from emp a left join emp b on a.mgr = b.empno;
+--------+-------+
| 员工 | 领导 |
+--------+-------+
| SMITH | FORD |
| ALLEN | BLAKE |
| WARD | BLAKE |
| JONES | KING |
| MARTIN | BLAKE |
| BLAKE | KING |
| CLARK | KING |
| SCOTT | JONES |
| KING | NULL |
| TURNER | BLAKE |
| ADAMS | SCOTT |
| JAMES | BLAKE |
| FORD | JONES |
| MILLER | CLARK |
+--------+-------+
14 rows in set (0.00 sec)
15.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.ename '员工',a.hiredate, b.ename '领导',b.hiredate,d.dname
from
emp a
join
emp b
on
a.mgr = b.empno
join
dept d
on
a.deptno = d.deptno
where
a.hiredate < b.hiredate;
+-------+------------+-------+------------+------------+
| 员工 | hiredate | 领导 | hiredate | dname |
+-------+------------+-------+------------+------------+
| CLARK | 1981-06-09 | KING | 1981-11-17 | ACCOUNTING |
| SMITH | 1980-12-17 | FORD | 1981-12-03 | RESEARCH |
| JONES | 1981-04-02 | KING | 1981-11-17 | RESEARCH |
| ALLEN | 1981-02-20 | BLAKE | 1981-05-01 | SALES |
| WARD | 1981-02-22 | BLAKE | 1981-05-01 | SALES |
| BLAKE | 1981-05-01 | KING | 1981-11-17 | SALES |
+-------+------------+-------+------------+------------+
16.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。
外连接
select
e.*,d.*
from
emp e
right join
dept d
on
e.deptno = d.deptno;
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | DEPTNO | DNAME | LOC |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO |
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 40 | OPERATIONS | BOSTON |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
17.列出至少有五个员工的所有部门
按照部门编号分组，计数，筛选出>=5
select
deptno,count(*)
from
emp
group by
deptno
having
count(*) >= 5;
+--------+----------+
| deptno | count(*) |
+--------+----------+
| 20 | 5 |
| 30 | 6 |
+--------+----------+
18.列出薪金比"SMITH"多的所有员工信息
select
ename,sal
from
emp
where
sal > (select sal from emp where ename = "SMITH");
+--------+---------+
| ename | sal |
+--------+---------+
| ALLEN | 1600.00 |
| WARD | 1250.00 |
| JONES | 2975.00 |
| MARTIN | 1250.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING | 5000.00 |
| TURNER | 1500.00 |
| ADAMS | 1100.00 |
| JAMES | 950.00 |
| FORD | 3000.00 |
| MILLER | 1300.00 |
+--------+---------+
19.列出所有"CLERK"（办事员）的姓名及其部门名称，部门的人数。
select
e.ename,d.dname,d.deptno
from
emp e
join
dept d
on
e.deptno = d.deptno
where
e.job = 'CLERK';
+--------+------------+--------+
| ename | dname | deptno |
+--------+------------+--------+
| MILLER | ACCOUNTING | 10 |
| SMITH | RESEARCH | 20 |
| ADAMS | RESEARCH | 20 |
| JAMES | SALES | 30 |
+--------+------------+--------+
// 每个部门的人数？
select
deptno,count(*) as deptcount
from
emp
group by
deptno;
+--------+-----------+
| deptno | deptcount |
+--------+-----------+
| 10 | 3 |
| 20 | 5 |
| 30 | 6 |
+--------+-----------+
// 将以上两张表当做两个t表
select
t1.*,t2.deptcount
from
(select e.ename,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;
+--------+------------+--------+-----------+
| ename | dname | deptno | deptcount |
+--------+------------+--------+-----------+
| MILLER | ACCOUNTING | 10 | 3 |
| SMITH | RESEARCH | 20 | 5 |
| ADAMS | RESEARCH | 20 | 5 |
| JAMES | SALES | 30 | 6 |
+--------+------------+--------+-----------+
20.列出最低薪金大于1500的各种工作及从事此工作的全部雇员人数.
select job from emp group by job having min(sal) > 1500;
+-----------+
| job |
+-----------+
| ANALYST |
| MANAGER |
| PRESIDENT |
+-----------+
select job,count(*) from emp group by job having min(sal) > 1500;
+-----------+----------+
| job | count(*) |
+-----------+----------+
| ANALYST | 2 |
| MANAGER | 3 |
| PRESIDENT | 1 |
+-----------+----------+
21.列出在部门“SALES”<销售部>工作的员工的姓名，假定不知道销售部的部门编号。
// 先查出来部门编号
select deptno from dept where dname= "sales";
// 再根据部门编号查员工的姓名
select ename from emp where deptno = (select deptno from dept where dname= "sales");
+--------+
| ename |
+--------+
| ALLEN |
| WARD |
| MARTIN |
| BLAKE |
| TURNER |
| JAMES |
+--------+
22. 列出薪金高于公司平均薪金的所有员工，所在部门，上级领导，雇员的工资等级。
// 注意自连接的时候left join
select
e.ename '员工',d.dname,e1.ename '领导',s.grade
from
emp e
join
dept d
on
e.deptno = d.deptno
left join
emp e1
on
e.mgr = e1.empno
join
salgrade s
on
e.sal between s.losal and s.hisal where
e.sal > (select avg(sal) from emp);
+-------+------------+-------+-------+
| 员工 | dname | 领导 | grade |
+-------+------------+-------+-------+
| JONES | RESEARCH | KING | 4 |
| BLAKE | SALES | KING | 4 |
| CLARK | ACCOUNTING | KING | 4 |
| SCOTT | RESEARCH | JONES | 4 |
| KING | ACCOUNTING | NULL | 5 |
| FORD | RESEARCH | JONES | 4 |
+-------+------------+-------+-------+
23.列出与"SCOTT"从事相同工作的所有员工及部门名称。
select job from emp where ename = "SCOTT";
+---------+
| job |
+---------+
| ANALYST |
+---------+
select
e.ename,e.job,d.dname
from
emp e
join
dept d
on
e.deptno = d.deptno
where
job = (select job from emp where ename = "SCOTT")
and
e.ename <> 'SCOTT';
+-------+---------+----------+
| ename | job | dname |
+-------+---------+----------+
| FORD | ANALYST | RESEARCH |
+-------+---------+----------+
24.列出薪金等于部门30中员工的薪金的其他员工的姓名和薪金。
// 部门30中的员工薪金
select distinct sal from emp where deptno = 30;
+---------+
| sal |
+---------+
| 1600.00 |
| 1250.00 |
| 2850.00 |
| 1500.00 |
| 950.00 |
+---------+
// 等于上面列表中的薪金且部门不为30
select ename,sal from emp where sal in (select distinct sal from emp where deptno = 30) and deptno <> 30;
Empty set (0.00 sec)
25.列出薪金高于在部门30工作的所有员工的薪金的员工姓名和薪金、部门名称。
select
max(sal)
from
emp
where
deptno = 30;
+----------+
| max(sal) |
+----------+
| 2850.00 |
+----------+
select
e.ename,e.sal,d.dname
from
emp e
join
dept d
on
e.deptno = d.deptno
where
e.sal > (select max(sal) from emp where deptno = 30);
+-------+---------+------------+
| ename | sal | dname |
+-------+---------+------------+
| KING | 5000.00 | ACCOUNTING |
| JONES | 2975.00 | RESEARCH |
| SCOTT | 3000.00 | RESEARCH |
| FORD | 3000.00 | RESEARCH |
+-------+---------+------------+
26.列出每个部门工作的员工人数，平均工资和平均服务期限。
没有员工的部门人数为0
注意是右连接
服务期限：系统当前年份 - 入职年份
select d.dname,d.deptno,count(e.ename),ifnull(avg(e.sal),0) avgsal
from
emp e
right join
dept d
on
e.deptno = d.deptno
group by
d.dname;
+------------+--------+----------------+-------------+
| dname | deptno | count(e.ename) | avgsal |
+------------+--------+----------------+-------------+
| ACCOUNTING | 10 | 3 | 2916.666667 |
| OPERATIONS | 40 | 0 | 0.000000 |
| RESEARCH | 20 | 5 | 2175.000000 |
| SALES | 30 | 6 | 1566.666667 |
+------------+--------+----------------+-------------+
在mysql当中怎么计算，两个日期的“年差”，差了多少年？
timestampdiff(间隔类型，前一个日期，后一个日期)
间隔类型：
SECOND 秒,
MINUTE 分钟,
HOUR 小时,
DAY 天,
WEEK 星期,
MONTH 月,
QUARTER 季度,
YEAR 年
select timestampdiff(YEAR,hiredate,now()) from emp;
+------------------------------------+
| timestampdiff(YEAR,hiredate,now()) |
+------------------------------------+
| 42 |
| 42 |
| 42 |
| 41 |
| 41 |
| 41 |
| 41 |
| 35 |
| 41 |
| 41 |
| 35 |
| 41 |
| 41 |
| 41 |
+------------------------------------+
select d.dname,d.deptno,count(e.ename),ifnull(avg(e.sal),0) avgsal,ifnull(avg( timestampdiff(YEAR,e.hiredate,now())),0) avgservicetime
from
emp e
right join
dept d
on
e.deptno = d.deptno
group by
d.dname;
+------------+--------+----------------+-------------+----------------+
| dname | deptno | count(e.ename) | avgsal | avgservicetime |
+------------+--------+----------------+-------------+----------------+
| ACCOUNTING | 10 | 3 | 2916.666667 | 41.0000 |
| OPERATIONS | 40 | 0 | 0.000000 | 0.0000 |
| RESEARCH | 20 | 5 | 2175.000000 | 38.8000 |
| SALES | 30 | 6 | 1566.666667 | 41.3333 |
+------------+--------+----------------+-------------+----------------+
*/
MySQL基础05 - 34道练习题
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转载自blog.csdn.net/weixin_43636084/article/details/129629408
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