Soft test --- database (1) database foundation

One 、 Database foundation

（1） data

        data （data） It is stored in the database The basic object , It's a symbolic record of things ;

（2） database

        database （ dataBase,DB ） It's stored in a computer for a long time 、 organized 、 A large amount of data that can be shared aggregate ;

        

        Database basic feature ：

        Data is organized according to a certain data model 、 Describe and store ;

        It can be shared by all kinds of users ;

        Little redundancy ;

        Data independence ;

        Easy to expand ; 

（3） Database management system

        Database management system （ DBMS ） It is a layer of data management software between the user and the operating system , It's a large and complex software system ;

         DBMS Use of ：

        Organize and store data scientifically

        Get and maintain data efficiently

（4） Database system

        Database system （ Database System ,DBS ） It is the system composition after introducing the database into the computer system ;

        

        The composition of the database system ：

        database ;

        Hardware platform ;

        Software （ Applications ）;

        Database administrator ;

        

        The design goal of database system is to allow users to process data logically , There is no need to involve how these data are stored in the computer , Provide a certain degree of independence between data organization and user applications ;

Two 、 database model

（1） Three modes of database system

        The database system can be divided into ：

        External mode （ Subscheme 、 User mode ）;

        Pattern （ Conceptual model 、 Logical mode ）;

        Internal mode （ Storage mode ）;

 （2） Pattern

        Description of logical structure and characteristics of all data in database ; 

        Common data view for all users , Integrated the needs of all users

        

        A database has only one schema ;

（3） External mode

        Database users （ Including application programmers and end users  ） Description of logical structure and characteristics of local data used ;

        Database user's data view , Is a logical representation of data related to an application ;

        

        The status of the external model ： Between mode and Application ;

        The relationship between patterns and patterns ： One to many ; 

        The external mode is usually A subset of patterns ;

        A database can have multiple external schemas , It reflects the application requirements of different users 、 How to look at data 、 Requirements for data confidentiality ;

        

        The use of external patterns ：

        A powerful measure to ensure the security of database ;

        Each user can only see and access the data in the corresponding external schema ;

 （4） Internal mode

        Is a description of the physical structure and storage of data ;

        Is the representation of data within the database ;

        A database has only one internal schema ; 

（5） Three levels

        Corresponding to the three-level mode , The database system can be divided into three levels of abstraction ：

        1. User level database ： Corresponding to external mode , It is the database that users see and use , Also known as user view , A database can have many different user views ;

        2. Concept level database ： Corresponding to the conceptual pattern , Is the minimum union of all user views , There is only one database application system DBA View ;

        3. Physical level database ： Corresponding to the inner mode , Is a low-level representation of the database , It describes the actual storage organization of data , Is closest to physical storage , Also known as internal view ;