31.多表查询之笛卡尔积-526互联

多表查询的定义

定义：通过查询多张表格获取数据，至少涉及两张表
数据准备：
- 创建部门表，插入三条数据
- 创建员工信息表添加外键约束，允许级联删除，并向三个部门插入对应的员工信息

## 创建部门信息表 
CREATE TABLE dept(  
id INT PRIMARY KEY AUTO_INCREMENT,  
dept_name VARCHAR(20),  
dept_manager VARCHAR(20),  
dept_location VARCHAR(20)  
); 
INSERT INTO dept VALUES(1,'研发部','张无忌','北京');
INSERT INTO dept VALUES(2,'运营部','赵敏','深圳');
INSERT INTO dept VALUES(3,'销售部','周芷若','成都');

# 创建员工信息表并添加级联删除的外键约束 
CREATE TABLE emp_part(  
emp_id INT PRIMARY KEY AUTO_INCREMENT,  
ename VARCHAR(20),  
age INT ,  
gender VARCHAR(10),
dept_id INT,
salary INT,
-- 添加外键约束 
CONSTRAINT emp_dept FOREIGN KEY(dept_id) REFERENCES dept(id)
-- 设置允许级联删除 
ON DELETE CASCADE
);

向员工信息表中插入数据

INSERT INTO emp_part VALUES(1,'令狐冲',25,'男','1',20000);
INSERT INTO emp_part VALUES(2,'任盈盈',23,'女','1',15000);  
INSERT INTO emp_part VALUES(3,'岳不群',45,'男','1',40000);
INSERT INTO emp_part VALUES(4,'任我行',40,'男','1',30000); 
INSERT INTO emp_part VALUES(5,'岳灵珊',21,'女','1',10000);
INSERT INTO emp_part VALUES(6,'赵灵儿',21,'女','2',7000); 
INSERT INTO emp_part VALUES(7,'林月如',22,'女','2',10000); 
INSERT INTO emp_part VALUES(8,'阿奴',20,'女','2',7000); 
INSERT INTO emp_part VALUES(9,'李逍遥',25,'男','2',15000); 
INSERT INTO emp_part VALUES(10,'景天',28,'男','2',20000);
INSERT INTO emp_part VALUES(11,'邱莹莹',21,'女','3',5000);
INSERT INTO emp_part VALUES(12,'关雎尔',22,'女','3',8000);
INSERT INTO emp_part VALUES(13,'曲筱绡',23,'女','3',10000);
INSERT INTO emp_part VALUES(14,'樊胜美',30,'女','3',10000);
INSERT INTO emp_part VALUES(15,'安迪',28,'女','3',20000);