1：等值连接
迪卡尔集连接
select ename, a.deptno as a_deptno,b.deptno as b_deptno ,b.dname as 部门
from emp a, dept b
等值连接
select ename, a.deptno as a_deptno,b.deptno as b_deptno ,b.dname as 部门
from emp a, dept b
where a.deptno = b.deptno;
2：非等值连接，在emp表和salgrade表中查找员工的姓名，工资，等级，工资上线，工资下线
select ename as 姓名, sal as 工资, grade as 工资等级,losal as 工资上线，hisal as 工资下线
from emp, salgrade
where sal between losal and hisal;
3:外连
右外连
select e.ename, d.dname, e.deptno
from emp e,dept d
where e.deptno = d.deptno(+);
左外连与右外连相反
以hr登陆查询表employees 和departments 表
select first_name as 姓名, department_name as 部门名称, d.department_id as 部门编号
from employees e, departments d
where e.department_id= d.department_id(+) ;
4:自连：在同一个表中查询每个员工及上司的工号和姓名
select a.empno as 员工编号, a.ename as 员工姓名, a.mgr as 上司的员工编号, b.ename as 上司姓名
from emp a, emp b
where a.mgr = b.empno;
5：sql99
交叉连接 cross join------相当于迪卡尔集
select e.ename,d.dname
from emp e cross join dept d;
自然连接 natural join------相当等值连接
select e.ename, d.dname
from emp e natural join dept d;
using子句-----使用同名列查询
select e.ename, d.dname
from emp e join dept d
using (deptno);
on子句 ------当列名不同时用on子句
用on查询两张表
select e.ename, d.dname
from emp e join dept d
on e.deptno = d.deptno;
用on查询多张表
select e.ename, d.dname
from emp e join dept d
on e.deptno = d.deptno
join 第三个表
on 列1 = 列2;
内连接（Inner join）-------内连接只返回满足连接条件的数据
select employee_id, last_name, salary, department_id, department_name
from employees inner join departments using(department_id);
左外连
select employee_id, last_name, salary, department_id, department_name
from employees left join departments using(department_id);
右外连
select employee_id, last_name, salary, department_id, department_name
from employees right join departments using(department_id);
满外连
select employee_id, last_name, salary, department_id, department_name
from employees full outer join departments using(department_id);
子查询------------
查找出工资比scott高的人
select ename, sal from emp
where sal>
(select sal from emp where ename='SCOTT');
查找那些人和scott相同职位的人
select ename, job from emp
where job=
(select job from emp where ename='SCOTT')
and ename <> 'SCOTT';
any的用法 < any意味着小于最大、> any大于最小
select empno, ename, sal, job
from emp
where sal<any(select sal from emp where job='SALESMAN');
all的用法 < all：小于所有，即小于最小、> all：大于所有，即大于最大
select empno, ename, sal, job
from emp
where sal<all(select sal from emp where job='SALESMAN');