重新学习数据库（3）

单元概述

通过本章的学习能够理解MySQL数据库中分组查询的含义，掌握常用分组函数的使用，掌握GROUP BY子句的使用规则，掌握分组后数据结果的条件过滤，掌握SELECT语句执行过程，理解子查询的含义，掌握单行子查询和多行子查询的使用

测试数据脚本：

drop TABLE emp;
drop TABLE dept;
drop TABLE salgrade;
create table dept
(
deptno integer(4) not null,
dname varchar(14),
loc varchar(13)
);
alter table dept add constraint pk_dept primary key (deptno);
create table emp
(
empno integer(4) not null,
ename varchar(10),
job varchar(9),
mgr integer(4),
hiredate date,
sal decimal(7,2),
comm decimal(7,2),
deptno integer(2)
);
alter table emp add constraint pk_emp primary key (empno);
alter table emp add constraint fk_deptno foreign key (deptno) references dept (deptno);
create table salgrade
(
grade integer(1),
losal decimal(7,2),
hisal decimal(7,2)
);
insert into DEPT (DEPTNO, DNAME, LOC) values (10, ‘ACCOUNTING’, ‘NEW YORK’);
insert into DEPT (DEPTNO, DNAME, LOC) values (20, ‘RESEARCH’, ‘DALLAS’);
insert into DEPT (DEPTNO, DNAME, LOC) values (30, ‘SALES’, ‘CHICAGO’);
insert into DEPT (DEPTNO, DNAME, LOC) values (40, ‘OPERATIONS’, ‘BOSTON’);
insert into SALGRADE (GRADE, LOSAL, HISAL) values (1, 700, 1200);
insert into SALGRADE (GRADE, LOSAL, HISAL) values (2, 1201, 1400);
insert into SALGRADE (GRADE, LOSAL, HISAL) values (3, 1401, 2000);
insert into SALGRADE (GRADE, LOSAL, HISAL) values (4, 2001, 3000);
insert into SALGRADE (GRADE, LOSAL, HISAL) values (5, 3001, 9999);
— 下面要特别注意，%d-%m-%Y 里的 d 和 m 一定要小写
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7369, ‘SMITH’, ‘CLERK’, 7902, STR_TO_DATE(’17-12-1980′, ‘%d-%m-%Y’), 800.00, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7499, ‘ALLEN’, ‘SALESMAN’, 7698, STR_TO_DATE(’20-02-1981′, ‘%d-%m-%Y’), 1600.00, 300.00, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7521, ‘WARD’, ‘SALESMAN’, 7698, STR_TO_DATE(’22-02-1981′, ‘%d-%m-%Y’), 1250.00, 500.00, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7566, ‘JONES’, ‘MANAGER’, 7839, STR_TO_DATE(’02-04-1981′, ‘%d-%m-%Y’), 2975.00, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7654, ‘MARTIN’, ‘SALESMAN’, 7698, STR_TO_DATE(’28-09-1981′, ‘%d-%m-%Y’), 1250.00, 1400.00, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7698, ‘BLAKE’, ‘MANAGER’, 7839, STR_TO_DATE(’01-05-1981′, ‘%d-%m-%Y’), 2850.00, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7782, ‘CLARK’, ‘MANAGER’, 7839, STR_TO_DATE(’09-06-1981′, ‘%d-%m-%Y’), 2450.00, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7788, ‘SCOTT’, ‘ANALYST’, 7566, STR_TO_DATE(’19-04-1987′, ‘%d-%m-%Y’), 3000.00, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7839, ‘KING’, ‘PRESIDENT’, null, STR_TO_DATE(’17-11-1981′, ‘%d-%m-%Y’), 5000.00, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7844, ‘TURNER’, ‘SALESMAN’, 7698, STR_TO_DATE(’08-09-1981′, ‘%d-%m-%Y’), 1500.00, 0.00, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7876, ‘ADAMS’, ‘CLERK’, 7788, STR_TO_DATE(’23-05-1987′, ‘%d-%m-%Y’), 1100.00, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7900, ‘JAMES’, ‘CLERK’, 7698, STR_TO_DATE(’03-12-1981′, ‘%d-%m-%Y’), 950.00, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7902, ‘FORD’, ‘ANALYST’, 7566, STR_TO_DATE(’03-12-1981′, ‘%d-%m-%Y’), 3000.00, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7934, ‘MILLER’, ‘CLERK’, 7782, STR_TO_DATE(’23-01-1982′, ‘%d-%m-%Y’), 1300.00, null, 10);

高级查询单元练习：

1. 查询部门20的员工，每个月的工资总和及平均工资。
   SELECT SUM(sal),AVG(sal)
   FROM emp
   WHERE deptno = 20;
2. 查询工作在CHICAGO的员工人数，最高工资及最低工资。
   SELECT COUNT(*),MAX(sal),MIN(sal)
   FROM emp e,dept d
   WHERE e.deptno = d.deptno
   AND d.loc = ‘CHICAGO’;
3. 查询员工表中一共有几种岗位类型。
   SELECT COUNT(DISTINCT job)
   FROM emp;

select job
from emp
group by job;

1. 查询每个部门的部门编号，部门名称，部门人数，最高工资，最低工资，工资总和，平均工资。
   SELECT d.deptno,d.dname,COUNT(*),MAX(e.sal),MIN(e.sal),SUM(e.sal),AVG(e.sal)
   FROM emp e
   JOIN dept d
   ON e.deptno=d.deptno
   GROUP BY deptno;
2. 查询每个部门，每个岗位的部门编号，部门名称，岗位名称，部门人数，最高工资，最低工资，工资总和，平均工资。
   select d.deptno,d.dname,e.job,count(*),max(e.sal),min(e.sal),sum(e.sal),avg(e.sal)
   from emp e,dept d
   where e.deptno=d.deptno
   group by d.deptno,d.dname,e.job;

3. 查询每个经理所管理的人数，经理编号，经理姓名，要求包括没有经理的人员信息。
   SELECT COUNT(*),m.empno,m.ename
   FROM emp e
   LEFT outer JOIN emp m
   ON e.mgr=m.empno
   GROUP BY m.empno;

4. 查询部门人数大于2的部门编号，部门名称，部门人数。
   SELECT d.deptno,d.dname,COUNT()
   FROM emp e,dept d
   WHERE e.deptno = d.deptno
   GROUP BY d.deptno
   HAVING COUNT()>2;

select d.deptno,d.dname,count()
from emp e
right outer join dept d
on e.deptno=d.deptno
group by d.deptno,d.dname
having count()>2;
2. 查询部门平均工资大于2000，且人数大于2的部门编号，部门名称，部门人数，部门平均工资，并按照部门人数升序排序
SELECT d.deptno,d.dname,COUNT(),AVG(sal)
FROM emp e,dept d
WHERE e.deptno = d.deptno
GROUP BY d.deptno
HAVING COUNT()>2 AND AVG(sal)>2000
ORDER BY COUNT(*) ASC;

select d.deptno,d.dname,count(),avg(e.sal)
from emp e
right outer join dept d
on e.deptno=d.deptno
group by d.deptno,d.dname
having avg(e.sal)>2000 and count()>2
order by count(*);

1. 查询入职日期最早的员工姓名，入职日期
   SELECT ename,hiredate
   FROM emp
   WHERE hiredate = (SELECT MIN(hiredate) FROM emp);
2. 查询工资比SMITH工资高并且工作地点在CHICAGO的员工姓名，工资，部门名称
   SELECT e.ename,e.sal,d.dname
   FROM emp e,dept d
   WHERE e.deptno = d.deptno
   AND e.sal>(SELECT sal FROM emp WHERE emp.ename = ‘SMITH’)
   AND d.loc = ‘CHICAGO’;
3. 查询入职日期比20部门入职日期最早的员工还要早的员工姓名，入职日期
   SELECT ename,hiredate
   FROM emp
   WHERE hiredate

Original: https://www.cnblogs.com/Aegeansea666/p/16484422.html
Author: Aegeansea666
Title: 重新学习数据库（3）