Java教程

数据库关系模型与实体关系模型

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 inconsistency
  • Data redundancy
  • Data integrity

Data models

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

  • Data
  • Data relationships
  • Data semantics
  • Data constraints

Categories:

  • Relational Model
  • Entity-Relationship Model

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 DMLDeclarative (nonprocedural) DML
Propertyuser instructs the system to perform a sequence of operations to obtain the dataUser describes the desired data
ExampleRelational-AlgebraSQL

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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