[打卡]内连接 VS 外连接

#SQL92内连接：
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.department_id = d.department_id;

SELECT * FROM employees;
SELECT * FROM departments;
#外连接：

#SQL92外连接不支持
SELECT last_name,department_name
FROM employees ,departments
WHERE employees.department_id = departments.department_id(+);

#SQL99内连接：
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.department_id = d.department_id;

SELECT employee_id,department_name
FROM employees e INNER JOIN departments d
ON e.department_id = d.department_id;

#多个内连接
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外连接：
#显示所有的员工和部门名
#左外连接（OUTER可省略）
SELECT employee_id,department_name
FROM employees e LEFT OUTER 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;