目录
一，多表查询的分类
角度1：等值连接 vs 非等值连接
角度2：自连接 vs 非自连接
角度3：内连接 vs 外连接
二，SQL92语法实现内连接：见上，略SQL92语法实现外连接：使用 + ----------MySQL不支持SQL92语法中外连接的写法！
三，SQL99语法如何实现多表的查询。
1，SQL99语法实现内连接：
2，SQL99语法实现外连接：
3，UNION 和 UNION ALL的使用
四，7种JOIN的实现：
1.中图：内连接
2.左上图：左外连接
3.右上图：右外连接
4.左中图：
5.右中图：
6.左下图：满外连接
方式1：左上图 UNION ALL 右中图
方式2：左中图 UNION ALL 右上图
7.右下图：左中图 UNION ALL 右中图
五，SQL99语法的新特性
1:自然连接
2:USING
一，多表查询的分类
角度1：等值连接 vs 非等值连接
#非等值连接的例子：
SELECT *
FROM job_grades;SELECT e.last_name,e.salary,j.grade_level
FROM employees e,job_grades j
#where e.`salary` between j.`lowest_sal` and j.`highest_sal`;
WHERE e.`salary` >= j.`lowest_sal` AND e.`salary` <= j.`highest_sal`;员工无法与等级相等，使用between and连接
角度2：自连接 vs 非自连接
SELECT * FROM employees;#自连接的例子
#练习：查询员工id,员工姓名及其管理者的id和姓名SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
FROM employees emp ,employees mgr
WHERE emp.`manager_id` = mgr.`employee_id`;员工和管理者不在一个表中，是非自链接
角度3：内连接 vs 外连接
内连接：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
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id; #只有106条记录外连接：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匹配的行之外，还查询到了左表 或 右表中不匹配的行。
外连接的分类：左外连接、右外连接、满外连接
左外连接：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。
右外连接：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。
#练习：查询所有的员工的last_name,department_name信息 SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id; # 需要使用左外连接二，SQL92语法实现内连接：见上，略
SQL92语法实现外连接：使用 + ----------MySQL不支持SQL92语法中外连接的写法！
#不支持：
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id(+);SQL99语法中使用 JOIN ...ON 的方式实现多表的查询。这种方式也能解决外连接的问题。MySQL是支持此种方式的。
三，SQL99语法如何实现多表的查询。
1，SQL99语法实现内连接：
SELECT last_name,department_name
FROM employees e INNER JOIN departments d
ON e.`department_id` = d.`department_id`;SELECT last_name,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`;2，SQL99语法实现外连接：
#练习：查询所有的员工的last_name,department_name信息
# 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：
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;#满外连接：mysql不支持FULL OUTER JOIN
SELECT last_name,department_name
FROM employees e FULL OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;3，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
结论：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。
四，7种JOIN的实现：
1.中图：内连接
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`;2.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`;3.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`;4.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;5.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;
6.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`;7.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;五，SQL99语法的新特性
1:自然连接：自动查询等值数据，自动连接，但不灵活，全部连接
SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
AND e.`manager_id` = d.`manager_id`;# NATURAL JOIN : 它会帮你自动查询两张连接表中`所有相同的字段`，然后进行`等值连接`。
SELECT employee_id,last_name,department_name
FROM employees e NATURAL JOIN departments d;
2:USING：用于替换等值连接，但不适用于非等值连接和自连接
SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
ON e.department_id = d.department_id;SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
USING (department_id);#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;
