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 int primary key auto_increment comment'ID',
name varchar(15) comment'姓名',
age int comment'年龄',
job varchar(15) comment '工作',
salary int comment '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,'2013-1-1', 1,1),
(3,'杨逍',13,'开发', 3000,'2021-1-1', 2,1),
(4,'纬一路',45,'开发',3450,'2024-1-2', 2,1),
(5,'常遇春',34,'开发',3020, '2016-1-1', 3,1),
(6,'小赵',23,'程序员鼓励师',3560,'2015-1-1', 2,1),
(7,'灭绝',26,'财务总监',3780,'2013-1-1', 1,3),
(8,'抽纸若',47,'会计',3230,'2023-1-1', 7,3),
(9,'金马',38,'出纳',3780,'2012-1-1', 7,3),
(10,'谢逊',56,'市场部总监',3034,'2010-1-1', 1,2),
(11,'大师',59,'职员',3550,'2009-1-1', 10,2),
(12,'白毛',50,'职员',3890,'2008-1-1', 10,2),
(13,'红爪',27,'职员',3220,'2007-1-1', 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,'2003-1-1', 14,4),
(17,'张三丰',53,'销售总监',6700,'2004-1-1', 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
