第一部分为啥要引入概念数据库

感觉只有一个重点实体联系模型----ER模型

第二部分-----实体联系模型

这个例子可以全看完之后再来看

举个例子:根据COMPANY数据库的需求来构造数据库模式：
The company is organized into DEPARTMENTs. Each department has a name, number and an employee who manages the department. We keep track of the start date of the department manager. A department may have several locations.
公司是由部门构成的。每个部门都有一个名字、编号和管理这个部门的员工。我们还需要记录每个部门经历开始管理这个部门的起始时间。一个部门可能有多个地点。
Each department controls a number of PROJECTs. Each project has a unique name, unique number and is located at a single location.
每个部门负责多个项目。每个项目有唯一的名称、唯一的编号并位于一个单独的位置。
We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate.
Each employee works for one department but may work on several projects.
我们存储每个员工的SSN，地址，薪资，性别和出生日期。每个员工都在一个部门工作但是可以参与多个项目。
We keep track of the number of hours per week that an employee currently works on each project.
We also keep track of the direct supervisor of each employee.
我们要记录每个员工在现阶段参与的项目上花费的时间，我们同时要记录每个员工的直接指导老师。
Each employee may have a number of DEPENDENTs.
For each dependent, we keep track of their name, sex, birthdate, and relationship to the employee.
每个员工可能有多个家属。对于每个家属，我们要记录他们的姓名，性别，生日和与员工的关系