注意：
一、多表查询的分类
1、等值连接 VS 非等值连接
非等值连接：只要不是==，其余条件都是非等值连接
2、自连接 VS 非自连接
3、内连接 VS 外连接
（1）内连接（INNER JOIN）
合并具有同一列的两个以上的表的行，结果集中不包含
一个表与另一个表不匹配的行。（满足两个表的关联条件才会被展示），inner可省略
（2）外连接(OUTER JOIN)
合并具有同一列的两个以上的表的行，结果集中除了包含
一个表与另一个表不匹配的行，还查询到了左表或右表中不匹配的行。
outer可省略
外连接的分类：左外连接，右外连接，满外连接。
左外连接(LEFT OUTER JOIN)
两个表在连接过程中除了返回满足连接条件的行以外还返回左表中不满足条件的行，这种连接称为左外连接。
SELECT last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;
右外连接(RIGHT OUTER JOIN)
两个表在连接过程中除了返回满足连接条件的行以外还返回右表中不满足条件的行，这种连接称为右外连接。
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
满外连接(FULL OUTER JOIN)
SELECT last_name,department_name
FROM employees e FULL OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;
MYSQL不支持FULL OUTER JOIN，使用union，示例见下
4、 UNION和UNION ALL的使用
union会执行去重操作，union all不会执行去重操作。如果明确知道合并数据后的结果数据不存在重复数据，或者不需要去除重复的数据，则尽量使用UNION ALL语句，以提高数据查询的效率。
5、七种SQL_JOINS的实现
中图：内连接
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`;
左上图： 左外连接
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;
右上图：右外连接
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;
左中图：
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL;
右中图：
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;
左下图：满外连接
方式1：左上图 UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;
方式2：左中图 UNION ALL 右上图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;
右下图：左中图 UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;
拓展：
SELECT last_name,job_title,department_name
FROM employees INNER JOIN departments INNER JOIN jobs
ON employees.department_id = departments.department_id
AND employees.job_id = jobs.job_id;
