Oracle Database Introduction

One 、 Database 4 Basic concepts

1） data (data)： Data is the basic object stored in the database
2） database （DataBase,DB)： The database is Long term storage In the computer , organized 、 Shareable A collection of large amounts of data
3） Database management system （DataBase Management System,DBMS)： Database management system is a layer of data management between users and operating system Systems software
4） Database system （DataBase System,DBS)： The database system consists of a database 、 Database management system 、 Application and database administrators （DBA) Composed storage 、 management 、 A system that processes and maintains data

Two 、 Two types of data models

The data model is Abstraction of real-world data features , In other words, the data model is used to describe data 、 Organizing and operating data . According to the different purposes of the model application , Divide the model into two categories .
The first category conceptual model ： Also known as information model , Modeling data and information from a user perspective , Mainly used for database design , The most commonly used representation is E-R chart （ Entity - How to contact ,Entity Relationship approach)
The second category logical model and The physical model ： The logical model mainly includes relational model 、 Hierarchical model and mesh model , Modeling data from the point of view of a computer system , It is mainly used for the realization of database management system ; Physical models are the lowest level abstraction of data , Describe the representation and access method of data in the system , Or storage mode and access method on disk or tape , It is oriented to computer system .
Data model 3 The elements
data structure ： Describe the constituent objects of the database and the relationship between objects ;
Data manipulation ： For various objects in the database （ type ） Example （ value ） A collection of allowed operations , Including operation and relevant operation rules ;
Data integrity constraints ： A set of integrity rules .
relational model
Relational model refers to a data model that represents entities and their relationships in the form of two-dimensional tables .
ps: Each component of a relationship must be an indivisible data item , That is, no table is allowed in the table .

3、 ... and 、 Three layer mode and two-layer image of database system

A schema is a description of the logical structure and characteristics of all data in a database , It only involves the description of type , No specific values are involved .
Three level mode
1） External mode ： Also known as subpattern or user mode , It's the database user's data view . External patterns are usually subsets of patterns , Therefore, there are multiple external modes , External mode DDL To define the outer mode .

2）（ Logic ） Pattern ： Description of logical structure and characteristics of all data in database , Is a common view for all users . A database has only one schema , Use mode DDL To define patterns .

3） Internal mode ： Also known as storage mode , A database has only one internal schema , It is the description of data structure and storage mode , It's how the data is organized inside the database .

In order to realize the connection and transformation of these three abstract levels within the system ,DBMS Provide a two-tier image between the three-level modes

Two layer image

External mode / Pattern image ： A pattern can have multiple external patterns . When （ Logic ） When the mode changes , from DBA Change the image accordingly , Without changing the external mode . The application is written according to the external mode , Therefore, there is no need to change , This is called logical independence of data .

Pattern / Internal mode image ： A database has only one internal schema . When the storage structure of the database changes , from DBA Change the image accordingly , Without changing the mode , Then there is no need to change the application , This is called the physical independence of data and program .

---

Reference books ： Introduction to Database System （ The fifth edition ） Wang Shan Huang,