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

-- 标量子查询
-- 案例 1.查询"销售部”的所有员工信息
-- a.查询“销售部"部门ID
select id  from  dept where name='销售部';
-- b 根据销售部部门ID，查询员工信息
select  *  from  staff where staff. dept_id=4;
select  *  from  staff where staff. dept_id=(select id  from  dept where name='销售部');

--  案例2.查询在"张三丰”入职之后的员工信息
-- a.查询在"张三丰”入职日期
select  wokeday from staff where name='张三丰';
-- b查询在"张三丰”入职之后的员工信息
select * from staff where wokeday>(select  wokeday from staff where name='张三丰');

案例1

案例2