关系模型¶
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Structure of Relational Databases¶
Basic Structure¶
关系用来指代表，元组用来指代行，属性用来指代列。
Attribute Types¶
- Each attribute of a relation has a name.
- The set of allowed values for each attribute is called the domain of the attribute.
- Attribute values are required to be atomic, i.e., indivisible
- multivalued attribute values are not atomic.(多值属性，例如手机号，一个人可能有多个手机号)
- composite attribute values are not atomic.(组合属性，例如地址)
- The special value null is a member of every domain
Concepts about Relaion¶
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concerned with two concepts: relation schema and relation instance
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The relation schema describes the structure of the domain.
- The relation instance corresponds to the snapshot of the data in the relation at a given instant in time.
类比
关系模式的概念对应于程序设计语言中类型定义的概念。 关系实例则对应于变量的值。给定变量的值可能随时间发生变化，当关系被更新时，关系实例的内容也随时间发生了变化。相反，关系模式一般不发生变化。
Relation Schema¶
Assume \(A_1,A_2,\cdots A_n\) are attributes, Formally expressed: $$ R=(A_1,A_2,\cdots A_n) $$ is a relation schema.
Eg: instructor=(ID,name,dept_name,salary)
\(r(R)\) is a relation on the relation schema \(R\).
Relation Instance¶
The Properties of Relation¶
- The order of tuples is irrelevant
- No duplicated tuples in a relation
- Attribute values are atomic
Key¶
- Let \(K\subset R\)
- \(K\) is a superkey(超码) of \(R\) if values for \(K\) are sufficent to identify a unique tuple of each possible relation \(r(R)\).
- \(K\) is a candidate key(候选码) if \(K\) is minimal superkey.
- \(K\) is primary key(主码) if \(K\) is a candidate key and is defined by user explicitly.(usually marked by underline)
Foreign Key
Schema Diagram¶
Relational-Algebra Operations¶
最后更新:
2024年3月18日 19:42:20
创建日期: 2024年3月18日 19:42:20
创建日期: 2024年3月18日 19:42:20