[学习笔记]数据库设计概览

一般来讲，设计一个数据库需要经过四个阶段：需求分析、概念设计、逻辑设计、物理设计。Note that there is no general agreement which defines these terms, nevertheless data modelers generally understand the approximate scope of each.

The conceptual model is concerned with the real world view and understanding of data;

The logical model is a generalized formal structure in the rules of information science;

The physical model specifies how this will be executed in a particular DBMS instance.

定义:A conceptual entity-relationship model shows how the business world sees information.

概念设计阶段要做如下三件事情

Constructing the ER Model

Check the model for redundancy

Validating the model against user transactions to ensure all the scenarios are supported

注意:

1) A conceptual model suppresses non-critical details in order to emphasize business rules and user objects.
2) It typically includes only significant entities which have business meaning, along with their relationships.
3) A conceptual model may include a few significant attributes to augment the definition and visualization of entities. No effort need be made to inventory the full attribute population of such a model.
4) A conceptual model may have some identifying concepts or candidate keys noted but it explicitly does not include a complete scheme of identity, since identifiers are logical choices made from a deeper context.

逻辑设计阶段要做两件事

Table Generation From ER Model

Normalization of Tables

注意:

1) 根据概念设计阶段的到的E-R图来生成具体的数据表时，要考虑E-R图中entity之间的对应关系怎样转换成数据表的对应关系。E-R图中的不同entity之间可能存在如下三种关系

one-to-one: 在建表时，一对一的关系是用A表的主键作为外键指向B表的主键来实现。建议将一对一关系的两个实体合并成一个实体，即将A、B两个表合并成一个表

one-to-many:将A表中的某个“非unique”的字段作为外键指向B表的主键，以此来实现一对多关系

many-to-many:一般需要在A、B量表之外另建一张新表C来实现A、B之间的多对多关系

2) 数据表的规范化，即要使数据表满足各个范式，但通常只要满足前三范式就足够了。（关于范式,请参考《数据库设计三大范式与BCNF》）
3) 亦有人说,逻辑设计阶段并不要得出数据库中具体的数据表,而是得出更完整的entity示图,其实说这个示图是entity还是数据库中的table都是无妨的,无论怎样叫,这一步公认的关键点在于:每一个entity都要具备完整的attribute和identity,而不像上一步建立概念模型时只需要表示出主要的字段和键表达出entity之间的逻辑关系即可。另外logical data model不能有many-to-many的关系，要将conceptual design阶段的many-to-many关系转化成中间表(或说中间实体)来实现。

定义：The physical data model specifies implementation details which may be features of a particular product or version, as well as configuration choices for that database instance

E-R图并非只用来设计概念模型，实际上：There are three types of ERDs: the conceptual, the logical, and the physical, which support modeling of data in three levels of abstraction from conceptual data model to implementation of a specific database.