Database System - 2nd theory class : Database Theory 20/03/09

首先，再次强调这是一个双语教程，所以学起来就会比较困难（正常操作，所以对自己有点信心吧小同学

正文开始，先复习一下上节课的内容）

Review

DB : Database
DBMS : Database Management System
DBS : Database System
Database System / file system (advantages / drawbacks)
Users of database system (five kinds of user)
Data model of database (Relational / Network / Hieriarchical / …）
View of Data(three levels includes View / Logical / Physical)
Database Language
Transaction Management
Query processing
History of database & challenges

Relational Model
the relational model was abstracted from table and processing methods of table .
SQL was proposed and developed based on relational model . It is the foundation of many database theories .
==>A relation is a table .

Domain (域) the range of values of Column / Attribute
the number of elements in the set is called Cardinality (基数) .

Cartesian product (笛卡尔积)
Formally , given sets of domain D1 , D2 , … , Dn , and the cartesian product is represented as :
D1 X D2 X … X Dn = { ( d1 , d2 , … , dn ) | di ∈ Di， i = 1，2，…，n}

The element of cartesian product , ( d1 , d2 , … , dn ) is called n-tuple (n元组) .
Each di in the tuple ( d1 , d2 , … , dn ) is called a component (分量) .
If the cardinality of Di is mi , then the cardinality if cartesian product .
i.e. the number of tuples is : m1 X m2 X … X mn
Formally , a relation r is a set of n-tuple ( d1 , d2 , … , dn ) where each di ∈ Di .
Not all tuples are meaningful .

Column Name / Attribute Name

Relation is denoted as R(A1:D1 , A2:D2 , … , An:Dn) , or briefly denoted as R(A1 , A2 , … , An) and we also call this kind of relation as Relation Schema .
R is the name of relation , Ai is the attribute , Di is the domain of attribute , n is the degree of relation ,the number of tuple in relation is called Cardinality of Relation .

Relation (Relation Instance 实例) and Relation Schema
One relation schema → many different relations (relation instances) .
Relation schema is the structure of relation , and relation (relation instances) is a snasphot of the data relation schema at a given instant in time .

关系的五个特性（ppt放太快没记下来英文就很尴尬）

Homogeneity (同质) of the column / attribute
Each component of one column comes from the same domain , and has a same data type .
The order in which columns appear in a relation is irrelevant (无关，不相关) .
The order in which tuples appear in a relation is irrelevant .
Each tuple / row should be different from other , i.e. no two tuples in a relaction are allowed to have exactly the same value for all attributes .
Attribute is atomic , and could not be divided anymore , and this character is also called NF .

Key : Candidate key [候选码 / 候选键]
Candidate key : an attribute or a group of attributes from a relation , its value or thier values can uniquely identify the tuple .
It is possible that there are more than one candidate key for a relation .

=======下次更新预告—白白白白，不定时，因为懒------------

Primary key & Super key [主码和超码]
Primary key : to denote a candidate key that is chosen by the database designers as the principal means of identifying the tuples within a relation .
Super key : a collection of attributes which includes candidate key .

Single key / All key & Primary attribute / non-peimary key [单一码/全码 & 主属性/非属性码]
single key :the candidate key has only one attribute .
All key :the group of all attributes which appears in any candidate key .
Primary attribute :the attribute which appears in any candidate key .
Non-primary attribute :other attribute in a relation except all primary attributes .

Foreign key [外码]
Foreign key :在R1中不是候选码，在R2中是候选码（在R1 ， R2中都存在）
其中R1–Referencing Relation R2–Referenced Relation

Integrity Constraint [完整性约束]

Entity Integrity : The attribute values of candidate key is not allowed to be NULL .
Referential Integrity (参照完整性)
User-defined Integrity : A kind of integrity which is defined by users according to different application .

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Database System - 2nd theory class : Database Theory 20/03/09

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