# Relational Model

## 基本结构

• 一张表对应一个关系(relation)，行对应元组(tuple)，列对应属性(attribute)
• 码 key
• Let $$K \subseteq R$$
• $$K$$ is a superkey of $$R$$ is values for $$K$$ are sufficient to identify a unique tuple of each possible relation
• $$K$$ is a condidate key is $$K$$ is minial superkey
• $$K$$ is a primary key, if K is a candidate key and is defined by user explicitly
• Foreign key Assume there exists relations $$r$$ and $$s$$: $$r(A,B,C)$$, $$s(B,D)$$, we can say that attribute $$B$$ in relation $$r$$ is foreign key referencing $$s$$, and $$r$$ is a referencing relation, and $$s$$ is a referenced relation.

## SQL

SQL：结构化查询语言，分为DDL,DML,DCL几种类型，详见SQL