[打卡]陈家昌 多表查询

#角度3：内连接vs外连接

SELECT e.employee_id,d.department_name
FROM employees e,departments d
WHERE e.department_id=d.department_id;
#查询所有，不能运行
SELECT e.employee_id,d.department_name
FROM employees e,departments d
WHERE e.department_id=d.department_id
#sql99内连接，查询员工id和部门名字
SELECT employee_id,department_name
from employees e JOIN departments d
on e.department_id = d.department_id;
#加一个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;
#OUTER可以省略掉
#左外
SELECT employee_id,department_name
from employees e left JOIN departments d
on e.department_id = d.department_id;
#右外
SELECT employee_id,department_name
from employees e RIGHT JOIN departments d
on e.department_id = d.department_id;