崔一博 多表查询 新

SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.department_id=d.department_id;
#查询员工id和部门名
SELECT * FROM employees;
#角度3：内连接vs外连接
#咱们之前写的都是内连接
#可能有员工没有部门，也可能有部门但是没有员工
#如果想把除了内连接的显示出来之外，还想把其他内容显示出来，即外连接
#外连接的分类：左外连接、右外连接、满外连接
#查询所有员工id和部门名107
#外连接涉及两个语法规范：一个是SQL92,一个是SQL99
#先来看SQL92
#SQL92内连接 见上
#SQL92外连接
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.department_id=d.department_id(+);
#以上SQL92外连接的语法在MySQL里是不能用的
#先来看SQL99
#SQL99的内连接
#查询员工id和部门名
SELECT employee_id, department_name
FROM employees e JOIN departments d
on e.department_id=d.department_id;
#也可以在join前加个inner，可以省略
SELECT employee_id, department_name
FROM employees e INNER JOIN departments d
on e.department_id=d.department_id;
#查询员工id和部门名和city
SELECT employee_id,department_name,city
FROM employees e join departments d
on e.department_id=d.department_id
JOIN locations l
on d.location_id=l.location_id;
#SQL99的外连接
#左外连接
#查询所有员工id和部门名107
SELECT employee_id,department_name
FROM employees e left OUTER join departments d
on e.department_id=d.department_id;
#outer可以省略掉
SELECT employee_id,department_name
FROM employees e left join departments d
on e.department_id=d.department_id;
#右外连接
SELECT employee_id,department_name
FROM employees e RIGHT join departments d
on e.department_id=d.department_id;
SELECT 122-106;