【MySQL】学习多表查询和笛卡尔积

-- 准备数据 create table dept( id int auto_increment comment 'ID' primary key , name varchar(50) not null comment '部门名称' )comment '部门表'; insert into dept(id, name) VALUES (1,'研发部'),(2,'市场部'),(3,'财务部'),(4,'销售部'),(5,'总经办'),(6,'人事部'); create table emp( id int auto_increment comment 'ID' primary key , name varchar(50) not null comment '姓名', age int comment '年龄', job varchar(20) comment '职位', salary int comment '薪资', entrydate date comment '入职时间', managerid int comment '直属领导ID', dept_id int comment '部门ID' )comment '员工表'; insert into emp (id, name, age, job, salary, entrydate, managerid, dept_id) VALUES (1,'金庸',66,'总裁',2000,'2000-01-01',null,5),(2,'张无忌',20,'项目经理',12500,'2005-12-05',1,1), (3,'杨逍',33,'开发',8400,'2000-11-03',2,1),(4,'韦一笑',48,'开发',11000,'2002-02-05',2,1), (5,'常遇春',43,'开发',10500,'2004-09-07',3,1),(6,'小昭',19,'程序员鼓励师',6600,'2004-10-12',2,1), (7,'灭绝',60,'财务总监',8500,'2002-09-12',1,3),(8,'周芷若',19,'会计',48000,'2006-06-02',7,3), (9,'丁敏君',23,'出纳',5250,'2009-05-13',7,3),(10,'赵敏',20,'市场部总监',12500,'2004-10-12',1,2), (11,'鹿杖客',56,'职员',3750,'2006-10-03',10,2),(12,'鹤笔翁',19,'职员',3750,'2007-05-09',10,2), (13,'方东白',19,'职员',5500,'2009-02-12',10,2),(14,'张三丰',88,'销售总监',14000,'2004-10-12',14,4), (15,'俞莲舟',38,'销售',4600,'2004-10-12',14,4),(16,'宋远桥',40,'销售',4600,'2004-10-12',14,4), (17,'陈友谅',42,null,2000,'2011-10-12',1,null); -- 添加外键将两张表进行关联 alter table emp add constraint fk_emp_dept_id foreign key (dept_id) references dept(id);