数据库系统概论——data model
Model
is the abstraction of real world.
模型是真实世界的抽象。
reveal the essence of objects, help people to locate and resolve problems
Data Model
explicitly determines the structure of data, and defines the operation that can be imposed, in order to represent the characteristic of the world.
数据模型定义了数据的结构和可执行的操作，以展示出现实世界的特性。
Data model is the core and fundamental of database
Data model determines the function and performance of database
There exists different data models
From different level, or different point of view
The evolution of data model promotes the database progress
Different level of abstraction
Data models
Conceptual model:
documents and organizes the data for communication between functional and technical people
概念模型用文档和数据用于功能和技术人员的交流。
Logical model:describes data semantics, data relationships, data constraints and data operation in database
逻辑模型描述了数据库中数据的语义，数据的关系，数据的约束和数据的操作
describes how data is orgnized in storage device
物理模型描述了存储设备上数据的组织方式。
Steps in Building a DB Application
概念设计——逻辑设计——物理设计
ER Model
is the most popular conceptual model
ER模型是最流行的一种概念模型
elements of ER model:
Entities, attributes, relationships
Entities
real-world objects distinguishable from other objects
described using a set of attributes
Entity set
a collection of similar entities
实体与实体集的关系如图。
Attributes
each has an atomic domain: string, integers, reals, etc.
For example：
产品（product）是一个实体集，具体的产品（specific product ）是实体。
price, name, category 是属性。
Relations
A mathematical definition:
if A, B are sets, then a relation R is a subset of A × B. (笛卡儿积)
Relationship Attribute
An attribute can also be property of a relationship.
For instance, the attribute access-date
ER Relationship Diagrams
Alternative notation for cardinality Constraints
下面是一些关于ER图、三元关系的例子：
三元联系：
三元关系转化成二元关系：