数据库关系模型与实体关系模型

2021/12/23 19:10:34

编程Tag： data 数据库 name 模型 model 关系 instructor Relational

本文主要是介绍数据库关系模型与实体关系模型，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

Database Systems

Purpose：provide users with an abstract view of the data

Data models

A collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints

Categories:

Data abstraction

Hide the complexity of data structures to represent data in the database from users through several levels of data abstraction

Data Manipulation Language (DML)

Language for data retrieval, insertion, deletion, modification

	Procedural DML	Declarative (nonprocedural) DML
Property	user instructs the system to perform a sequence of operations to obtain the data	User describes the desired data
Example	Relational-Algebra	SQL

SQL:
create table instructor (
ID char(5),
name varchar(20),
dept_name varchar(20),
salary numeric(8,2))

select	name from instructor	where	instructor.dept_name = ‘History’

Relational Algebra:

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