MySQL--day5--多表查询

（以下内容全部来自上述课程）

多表查询

1. 为什么要用多表查询

# 如果不用多表查询
#查询员工名为'Abel'的人在哪个城市工作? 
SELECT*
FROM employees
WHERE last_name = 'Abel';

SELECT *
FROM departments
WHERE department_id = 80;

SELECT *
FROM locatipns
WHERE location_id=2500;

2. 多表查询如何实现

2.1 出现笛卡尔积错误

# 2. 出现笛卡尔积错误（交叉连接）
# 错误的实现方式:每个员工都与每个部门匹配了一遍。
# 错误原因：缺少了多表的连接条件 
SELECT employee_id,department_name
FROM employees,departments; # 查询出2889条记录

SELECT *
FROM employees; # 107条记录

SELECT 2889 / 107 
FROM DUAL;

SELECT *
FROM departments; # 27条记录

2.2 正确方式

#3.多表查询的正确方式:需要有连接条件
SELECT employee_id,department_name 
FROM employees,departments
#两个表的连接条件
WHERE employees.department_id = departments.department_id;

2.3 指明字段（优化）

#4.如果查询语句中出现了多个表中都存在的字段，则必须指明此字段所在的表。
SELECT employees.employee_id,departments.department_name, employees.department_id 
FROM employees,departments
WHERE employees.`department_id`= departments.department_id;
#建议:从sql优化的角度，建议多表查询时，每个字段前都指明其所在的表。

2.4 别名

#5.可以给表起别名，在SELECT和WHERE中使用表的别名。
SELECT emp.employee_id, dept.department_name, emp.department_id 
FROM employees emp,departments dept
WHERE emp.department_id = dept.department_id;

#如果给表起了别名，一旦在SELECT或WHERE中使用表名的话，则必须使用表的别名，而不能再使用表的原名。 
# 如下的操作是错误的：
SELECT emp.employee_id,departments.department_name,emp.department_id 
FROM employees emp,departments dept
WHERE emp.department_id = departments.department_id;

2.5 练习

#6.如果有n个表实现多表的查询，则需要至少n-1个连接条件
#练习:查询员工的employee_id,last_name,department_name,city
SELECT e.employee_id,e.last_name,d.department_name,l.city,e.department_id,l.location_id 
FROM employees e,departments d,locations l 
WHERE e.`department_id` = d.`department_id` 
AND d.`location_id`= l.`location_id`;

3. 多表查询的分类

3.1 等值连接 vs 非等值连接

上述例子都是等值连接

#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`;

3.2 自连接 vs 非自连接

#7.2 自连接	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`;

3.3 内连接 vs 外连接

#7.3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 
#        就是两表交集
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。 
#       就是量表并集	
# 外连接的分类:左外连接（交集+e左部分）、右外连接（交集+d右部分）、满外连接（并集）具体看下方图片
# 查询"所有"就是外连接。

3.4 SQL92语法

#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	

#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!

# 左连接：左腿长右腿短，垫右腿
SELECT employee_id,department_name 
FROM employees e,departments d
WHERE e.`department_id` = d.department_id(+);

3.5 SQL99语法

#SQL99语法实现内连接:
SELECT last_name,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`;

#JOIN一个表ON一个条件
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`;

#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 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`;

4. SQL99的7种JOIN操作

4.1 UNION操作符

• UNION ALL 比 UNION 多了一倍重叠的部分
• 满外连接需要用UNION
• 开发中能用UNION ALL的时候不用UNION
• 无重复部分用UNION ALL效率更高

4.2 7种实现

#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.`departmentid`;

#左中图: 左上图+右null条件（中间挖出去）
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: 右上图+左null条件（中间挖出去）
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 eI LEFT JOIN departments d 
ON e.`department id` = d.`department id` 

UNION ALL

SELECT employee_id,department_name
FROM employees eRIGHT 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 eRIGHT JOIN departments d 
ON e.`department_id`= d.`department_id` 
WHERE e.`department_id` IS NULL;

5. SQL99新特性

5.1 自然连接

#10. SQL99语法的新特性1:自然连接
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`;

SELECT employee_id,last_name,department_name 
FROM employees e NATURAL JOIN departments d;

5.2 USING连接

#11.SQL99语法的新特性2:USING
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);	I