[打卡]Aloe0416 --多表查询3

-- 内连接
#在之前写的一般都是内连接
-- 如果想把除了内连接的显示出来，还想把其他的内容显示出来，即为外连接
-- 外连接分为左外连接，右外连接和满外连接
-- 外连接一般有两种语法规范 SQL92 SQL99

-- SQL92的内连接即为以前学习过的
-- SQL92的外连接
SELECT e.employee_id,d.department_name
FROM employees e,departments d
WHERE e.department_id,d.department_id(+);
-- SQL92的外连接的语法在Mysql中是运行不了的

-- SQL99的内连接 (INNER可以省略)
SELECT employee_id,department_name
FROM employees e INNER JOIN departments d
ON e.department_id = d.department_id;
-- SQL99三个及以上的内连接
SELECT employee_id,department_name
FROM employees e INNER 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
-- 1.左外连接 (OUTER可以省略)
SELECT employee_id,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.department_id = d.department_id;
-- 2.右外连接 (OUTER可以省略)
SELECT employee_id,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.department_id = d.department_id;