create table dept(id int primary key auto_increment,name varchar(15))comment '部门';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 staff(id intprimary key auto_incrementcomment'ID',name varchar(15) comment'姓名',age int comment'年龄',job varchar(15) comment '工作',salary intcomment '薪资',wokeday date comment '工作时间',mange int comment '直属领导Id',dept_id int comment '部门Id')comment '员工';insert into staff ( id,name,age,job,salary,wokeday,mange,dept_id)values(1,'金庸',66,'总裁',3000,'2000-1-1',null,5),(2,'张无忌',34,'项目经理',3000,'-1-1', 1,1),(3,'杨逍',13,'开发',3000,'-1-1', 2,1),(4,'纬一路',45,'开发',3450,'2024-1-2',2,1),(5,'常遇春',34,'开发',3020, '-1-1', 3,1),(6,'小赵',23,'程序员鼓励师',3560,'-1-1', 2,1),(7,'灭绝',26,'财务总监',3780,'-1-1',1,3),(8,'抽纸若',47,'会计',3230,'-1-1', 7,3),(9,'金马',38,'出纳',3780,'-1-1', 7,3),(10,'谢逊',56,'市场部总监',3034,'-1-1', 1,2),(11,'大师',59,'职员',3550,'-1-1', 10,2),(12,'白毛',50,'职员',3890,'-1-1', 10,2),(13,'红爪',27,'职员',3220,'-1-1', 10,2),(14,'龙珠',34,'销售总监',4000,'2001-1-1',1,4),(15,'宋远桥',45,'销售总监',5000,'2002-1-1', 14,4),(16,'天王例',89,'销售总监',8900,'-1-1', 14,4),(17,'张三丰',53,'销售总监',6700,'-1-1', 1,null);alter table staff add constraint fk_staff_dept_id foreign key (dept_id) references dept(id) ; -- 建立外键链接案例1-- 多表查询select * from staff,dept;-- staff表有17条，dept表有6条 ，6*17=102条数据案例2-- 多表查询（消除无效的笛卡尔积）select *from staff,dept where staff.dept_id=dept.id;-- 因为张三丰这个员工没有安排部门，所以不满足 staff.dept_id=dept.id这个条件，所以查询不出来
案例1
案例2
