数据库关系模型与实体关系模型
作者：互联网
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 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:
标签：关系,name,data,模型,Relational,Model,instructor,Data,数据库 来源： https://blog.csdn.net/weixin_46054349/article/details/122104757