Dept（部门表）

Emp（员工表）

SALGRADE（工资等级表）

Bobus（工资条）

SELECT

查询表中所有的数据
select * from dept;

查询某一列或某几列
select deptno from dept;
select deptno, dname from dept;

给查询结果的列起别名
select deptno dno, dname from dept;
select deptno 部门编号, dname from dept;
select deptno "部门 编号", dname from dept;

带计算式
select empno, ename, sal*12 from emp;

拼接字符串
select ename||'-entor' from emp;
select ename||' '||job from emp;

去除重复
select distinct deptno from emp;
select distinct deptno, job from emp;

WHERE

关系运算符 =, >, <, >=, <=, !=(<>)
select * from emp where empno=7934;

select * from emp where job='CLERK';

SELECT * FROM emp where sal > 1000;

select * from emp where deptno<>30;

select * from emp where hiredate>'02-4月 -81';

SELECT * FROM emp where sal*12 > 10000;

逻辑运算符 and, or, not
 select * from emp where deptno=20 and sal>1000;
select * from emp where deptno=20 or sal>1000;
select * from emp where not sal>1000;

select * from emp where (deptno=20 and sal>1000) or sal<1000;

范围运算符 between...and
select * from emp where sal>=1000 and sal<=2000;
select * from emp where sal between 1000 and 2000;

谓词运算符 in
select * from emp where empno=7499 or empno=7521 or empno=7654;
select * from emp where empno in (7499, 7521, 7654);

空判断 is null, is not null
select * from emp where comm is null;
select * from emp where comm is not null;

取出所有员工的月收入（工资+提成）
select empno, sal, comm, sal+comm from emp;  --计算式中包含null，结果一定是null

select empno, comm from emp where comm in (300, 500);
select empno, comm from emp where comm not in (300, 500); --in和not in的结果都不包含null
select empno, comm from emp where comm not in (300, 500, null);  --not in不能有null，否则所有行都被过滤

模糊查询Like
select ename from emp where ename like 'J%';
select ename from emp where ename like '%T%';

Order By
select empno, sal from emp order by sal;
select empno, sal from emp order by sal desc;

多个字段排序
select empno, sal from emp order by sal desc, empno asc;

select empno, sal from emp where sal>1000 order by sal desc;

组函数
sum, avg, max, min, count
select sum(sal) from emp;
select avg(sal) from emp;
select max(sal) from emp;
select min(sal) from emp;
select sum(sal) from emp where deptno=10;

count
select count(*) from emp;
select count(1) from emp;
select count(comm) from emp;  --针对某列计数，null不算有效行
select count(deptno) from emp;
select count(distinct deptno) from emp;  --先去重复，再计数

分组Group By
select deptno, sum(sal) from emp group by deptno;
select deptno, job, max(sal) from emp group by deptno, job;
select deptno, job,sal, max(sal) from emp group by deptno, job;  --会报错，sal字段不在group by 子句中，不能在select中出现

分组后的结果进行过滤Having
部门总工资高于7000的才显示
select deptno, sum(sal) from emp group by deptno having sum(sal)>7000;

多表查询
等值连接

--显示员工的姓名和部门名称
select ename,dname from dept, emp where dept.deptno=emp.deptno

不等值连接
--显示员工和他工资对应的工资等级
select emp.ename, sal, grade from emp, salgrade 
where emp.sal >= salgrade.losal
and emp.sal <= salgrade.hisal

表的别名
select d.deptno 
from dept d, emp e
where d.deptno=e.deptno;

--显示员工的姓名和部门名称和他工资对应的工资等级
select ename, dname, grade
from emp, dept, salgrade
where emp.deptno = dept.deptno
and emp.sal between salgrade.losal and salgrade.hisal

自连接
--显示员工姓名和他直接上级的姓名
select e1.ename, e2.ename
from emp e1, emp e2
where e1.mgr = e2.empno

JOIN
内连接 inner join
select * from emp e join dept d on d.deptno=e.deptno
where e.sal > 1000
and e.comm is not null

select * from emp e 
join dept d on d.deptno=e.deptno
join salgrade g on e.sal between g.losal and g.hisal
where e.sal > 1000
and e.comm is not null

select e1.ename, e2.ename
from emp e1
join emp e2 on e1.mgr = e2.empno

外连接
左外连接
select e.ename, d.dname from emp e  left join dept d on d.deptno=e.deptno

右外连接
select e.ename, d.dname from emp e  right join dept d on d.deptno=e.deptno

全外连接
select e.ename, d.dname from emp e  full join dept d on d.deptno=e.deptno

列出员工数量多于3人的部门名称及人数
select dept.dname,count(*) 
from emp 
join dept on emp.deptno=dept.deptno
group by dept.dname 
having count(*) > 3

列出各部门的平均工资和部门名称（没有员工的部门也要列出）
select dept.dname, avg(sal)
from emp 
right join dept on emp.deptno=dept.deptno
group by dept.dname 

子查询
列出工资最高的员工姓名及工资
select * from emp where sal=(select max(sal) from emp)

列出工资高于平均工资的员工信息
select * from emp where sal>(select avg(sal) from emp)

列出部门平均工资高于2000的所有部门的员工
select * from emp where deptno in 
(select deptno 
from emp 
group by deptno 
having avg(sal)>2000)

列出每个部门内工资最高的员工
select emp.* from emp, 
(select deptno, max(sal) maxsal from emp group by deptno) t
where emp.deptno=t.deptno
and emp.sal=t.maxsal

列出每个部门的平均工资对应的工资等级
select deptno,grade from 
(select deptno, avg(sal) avgsal from emp group by deptno) t
join salgrade on t.avgsal between losal and hisal