一、dept和emp表结构：

create table dept

(

deptno int(100) not null,

dname varchar(255),

loc    varchar(255),

);

alter table dept add constraint PK_DEPT primary key (deptno);

create table emp

(

empno    int(100) not null,

ename    varchar(255),

job varchar(255),,

mgr       int(100),

hiredate date,

sal       int(100),

comm      int(100),

deptno    int(100)

);

alter table EMP add constraint PK_EMP primary key (empno);

二、初始化数据：

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');

commit;

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7369, 'SMITH', 'CLERK', 7902, str_to_date('1980-12-17', '%Y-%m-%d'), 800, null, 20);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7499, 'ALLEN', 'SALESMAN', 7698, str_to_date('1981-02-20', '%Y-%m-%d'), 1600, 300, 30);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7521, 'WARD', 'SALESMAN', 7698, str_to_date('1981-02-22', '%Y-%m-%d'), 1250, 500, 30);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7566, 'JONES', 'MANAGER', 7839, str_to_date('1981-04-02', '%Y-%m-%d'), 2975, null, 20);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7654, 'MARTIN', 'SALESMAN', 7698, str_to_date('1981-09-28', '%Y-%m-%d'), 1250, 1400, 30);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7698, 'BLAKE', 'MANAGER', 7839, str_to_date('1981-05-01', '%Y-%m-%d'), 2850, null, 30);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7782, 'CLARK', 'MANAGER', 7839, str_to_date('1981-06-09', '%Y-%m-%d'), 2450, null, 10);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7788, 'SCOTT', 'ANALYST', 7566, str_to_date('1987-04-19', '%Y-%m-%d'), 3000, null, 20);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7839, 'KING', 'PRESIDENT', null, str_to_date('1981-11-17', '%Y-%m-%d'), 5000, null, 10);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7844, 'TURNER', 'SALESMAN', 7698, str_to_date('1981-09-08', '%Y-%m-%d'), 1500, 0, 30);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7876, 'ADAMS', 'CLERK', 7788, str_to_date('1987-05-23', '%Y-%m-%d'), 1100, null, 20);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7900, 'JAMES', 'CLERK', 7698, str_to_date('1981-12-03', '%Y-%m-%d'), 950, null, 30);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7902, 'FORD', 'ANALYST', 7566, str_to_date('1981-12-03', '%Y-%m-%d'), 3000, null, 20);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7934, 'MILLER', 'CLERK', 7782, str_to_date('1982-01-23', '%Y-%m-%d'), 1300, null, 10);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7978, 'MILLER', 'CLERK', 7782, str_to_date('1982-01-23', '%Y-%m-%d'), 1300, null, 40);

insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)

values (7972, 'MIKER', 'CLERK', 7792, str_to_date('1982-07-23', '%Y-%m-%d'), 3300, null, 40);

commit;

三、题目练习

1．列出至少有三个员工的所有部门。

select deptno from emp group by deptno having count(*)>3;

解析：该语句主要学习分组函数group by，以及对分组后过滤的条件函数having。需要注意的是使用了分组函数语句的select字段中不能包含group by后没有的字段，原因是如果显示非分组的字段就可能显示多条记录，就达不到按某字段分组的目的。Having只能用在分组函数group by后，相当于对分组后的记录做where条件过滤。

2．列出薪金比“SMITH”多的所有员工。

select * from emp where sal>(select sal from emp where ename='SMITH');

解析：该语句主要学习子查询，且子查询在where条件的后面。注意和3的区别。

3．列出所有员工的姓名及其直接上级的姓名。

select ename ,(select ename  from emp  where empno=a.mgr ) from emp a;

解析：该语句主要学习子查询，且子查询在select 和 from的中间，同时内部语句使用了外部语句的字段。注意和2的区别。

4．列出受雇日期早于其直接上级的所有员工。

select ename from emp a where hiredate>(select hiredate from emp where empno=a.mgr)

解析：该语句主要学习子查询，且子查询在where条件的后面。同时内部语句使用了外部语句的字段。注意和2的区别。注意和2的区别。

5．列出最低薪金大于1500的各种工作。

select job,min(sal) minsal from emp group by job having minsal >1500

解析：该语句主要学习分组函数，功能同1。

6．列出在每个部门工作的员工数量、平均工资和平均服务期限。

select deptno,count(*)people ,avg(sal),avg(DATEDIFF(CURDATE(),hiredate))avgdays from emp group by deptno

解析：该语句主要学习分组函数，同时学习其相关的统计函数count, avg等。

本人技术小白，如有错误或需改进之处，请各位大佬帮忙指出！谢谢！