Vu Database 备考宝典

Week 2 - Database Design

1.Advantages of relational database？

Structural independence
Data needs storing once and independent of the application programs
Reduced data redundancy
Eliminate data anomalies
Increased Data Sharing
Many people can access the DBMS
Easy access
Ability to make ad hoc queries by using Structured Query Language (SQL)
Better security
Easy to provide varying access privileges
Easy to expand for future data growth by adding any tables

2.The role of PK

To uniquely identify each row.

3.Rules to chose a Primary Key

to be unique
Consider all possibilities of data values
Use the minimum number of columns if using combined columns.
Has to be a stable value – not often changing

4.The role of FK

Foreign key is useful to establish a relationship between two relations.

5.Rules to chose a Foreign Key

An attribute or a combination of attributes in one table whose values must either match the primary key in another table or be NULL .

6.Difference between database and DBMS(database management system )

A DBMS can contain one or more databases.
Each database will communicate with the database management system (DBMS) to process the contents of the database tables.

7.Top-down design
<1>Identification of different entity types
<2>Definition of each entity’s attributes
即先建实体表再填充表里的内容

8.Bottom-up design
<1>Defines the attributes
<2>Groups them to form entities

9.Conceptual data model（概念数据模型） $\rightarrow$ Logical data model（逻辑数据模型） $\rightarrow$ Physical model（物理数据模型）

概念数据模型是现实世界到信息世界的第一层抽象，简单定义几个实体去体现他们之间的联系
逻辑数据模型是对概念数据模型进一步具体化，在概念数据模型定义实体的基础上定义了各个实体的属性
物理数据模型是在逻辑数据模型的基础上，综合考虑各种存储条件的限制，进行数据库的设计，从而真正实现数据在数据库中的存放。其主要的工作是根据逻辑数据模型中的实体、属性、联系转换成对应的物理模型中的元素，包括定义所有的表和列，定义外键以维持表之间的联系等

10.SQL（ Structured Query Language）
SQL can be divide into 3 categories as DDL, DML or DCL

DDL （ Data Definition Language）
SQL can be used to describe the logical, and sometimes physical, structure of a database (Create, Alter and Drop table commands)
DML （Data Manipulation Language）
SQL is used to access and modify the contents (data) of a database (Select, Insert, Update, and Delete commands)
Transaction control DML statements are Commit and Rollback
DCL （Data Control Language）
SQL is used to control access to data (Grant and Revoke commands)

Week 3 - Data Modelling

1.one-to-one (1:1)

解析：
这个是ER图的1:1表现。可见两边都是||。变红的那个是作为外键。

2.one-to-many (1:M)

解析：
||还代表1，那个分叉代表M。红色的为外键。

3.many–to–many(M:N)

解析：
1：M：N：1即实现M：N

4.Data dictionary
内容：

The definitions for the data elements
The composition of data - Each entity and its associate attributes
Data kept

注意：
在Null（y/n）中，n表示不能为空，y表示可以为空。

Single Table Query

单表查询详解

Multiple Table Query

多表查询详解

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Normalization

1.What is database Normalisation?
Normalisation is a process that involves arranging attributes in a relation, based on functional dependencies among the attributes.

2.Benefits of normalization
Normalisation results in reducing data redundancy and improving data integrity.

3.Normalisation process

Top Down Modelling
Suitable for larger systems.
Identify entities, then relationships and attributes create a data model.
Bottom up Modelling（relational analysis）
Suitable for a small manageable system.
<1>Gathering all attributes concerned with a problem
<2>Creating one relation holding all attributes
<3>Applying the normalisation rules to this relation, breaking in down into smaller relations as duplications and dependencies

4.Full functional dependencies (FDD)
Attribute B is fully functionally dependent on composite attribute A, if is dependent on the whole of A, but not on any subset of A （说白了就是完全依赖于组合主键）
eg：

具体例题解析如下：
三范式

Physical Database Design

从分析数据到设计出物理数据库的的流程：

分开来看：
Logical database design：
1.Complete Logical data model

There should be NO many-to-many relationships - introduce associative entities where appropriate.
Add cardinality and optionality
Both Primary Keys (PK) and Foreign Keys (FK) are to be shown.

2.Draw Relations
3.Normalse relations

Physical database design：
1.Tables in a relational database
2.Physical implementation

Logical to physical conversion

Normalised relations in logical design becomes tables in the physical design （简单说就是将逻辑设计中的关系图具体化成数据库中的表）
Each attribute in a relation becomes a field in the corresponding table in the database
The conversion needs to pay attention to field names, descriptions, data types and size

Merging Relations（合并关系）
1.Synonyms（同义词可合并）

2.Homonyms（同音词不可合并）