Introduction to database system - Chapter 1 introduction [conceptual model, hierarchical model and three-level mode (external mode, mode, internal mode)]

1, Data system overview

Four basic concepts of database ：

data 、 database 、 Database management system 、 Database system ;

• 1 data ： The symbolic record that describes things is called data
• 2 Database is stored in computer for a long time 、 Organized 、 A collection of large amounts of data that can be shared .
• 3 Database management system is to solve how to efficiently acquire and maintain data 、 How to organize and store data scientifically
• 4 The database system is controlled by the database 、 data management system （ And its application development tools ）、 Storage composed of application and database administrators 、 management 、 A system that processes and maintains data .

The emergence and development of data management technology

• 1 Manual management stage ,2 File system phase ,3 Data system phase

Characteristics of database system

• 1 Data structure ： The database system realizes the structure of the whole data , This is one of the main features of the database , It is also the essential difference between database system and file system .
  （ The whole refers to that the data in the database is no longer only for a certain application , But for the whole organization or enterprise ）; It is not only internally structured but also integrally structured , There is a connection between data .
  2 High data sharing 、 Low redundancy and easy expansion ;
  （ Data sharing can also avoid incompatibility and inconsistency between data .）
  3 High data independence ：
  ① Physical independence means that the physical storage of data in user applications and databases is independent of each other .
  ② Logical independence means that the logical structure of the user's application and data are independent of each other .
  4 The data is uniformly managed and controlled by the data management system ;
  （1） Data security protection ;
  （2） Data integrity check ;
  (3) concurrency control ;
  （4） Data recovery ;
• summary ： The database is organized and stored in the computer for a long time 、 A lot of 、 Shared data collection . It can be shared by various users , It has minimum redundancy and high data independence . The database management system is established in the database 、 The database shall be under unified control during operation and maintenance , To ensure data integrity and security , And use the database for concurrency control in multiple users at the same time , Recover the database after failure .

2 Data model ： Data model is an abstraction of the characteristics of real-world data , Data model is the core and foundation of database system .

Two types of data models ;

• The first kind of conceptual model

  • Also known as the information model , Modeling data and information according to users' ideas is mainly used in database design .

• The second is the logical model and the physical model

conceptual model

• 1 The basic concepts in the information world ：
  ① Entity ： Things that exist objectively and can be distinguished from each other
  ② attribute ： An entity has a property called an attribute
  ③ code （key）： The set of attributes that uniquely identify the entity
  ④： Solid type ： Using entity name and its attribute name set to abstract and characterize similar entities , It's called the solid body ;
  ⑤ Entity set ： A set of entities of the same type is called an entity set .
  ⑥ contact ： The relationship between entities usually refers to the relationship between different sets of entities
• Entity - How to contact （E—R Model ）

The components of the data model ： Data model is usually composed of data structure 、 Data operation and data integrity constraints are three parts .

• data structure ： Describe the objects that make up the database and the relationships between them .
  Data manipulation ： It refers to the collection of operations allowed for instances of various objects in the database , Including operation and relevant operation rules .
  Data integrity constraints ： Data integrity constraints are a set of integrity rules

Common data models

• hierarchical model

  • 1 There is only one node without parents , This node is called the root node ;
    2 Nodes other than the root have only one parent node .
    The hierarchical model is like an inverted tree , Node parents are unique .
    

• Mesh model

  • Data structure of mesh model ：
    （1） Allow more than one node to have no parents .
    （2） A node can have more than one parent .
  • 

• relational model ： The data structure of each relationship is a normalized two-dimensional table

  • Some terms of relational model ：
    ① Relationship （relation）： Represents a table corresponding to the usual ;
    ② Tuples （tuple）： A row in the table is a tuple ;
    ③ attribute （attribute）： A column in the table is an attribute ;
    ④ code （key）： Also called code key .
    ⑤ Domain （domain）： A field is a set of values of the same data type ;
    ⑥ component ： An attribute value in a tuple .
    ⑦ Relationship model ： Description of the relational model , It is generally expressed as ：
    Relationship name （ attribute 1, attribute 2,···, attribute n）
  • The relationship model requires that the relationship must be normalized , Each component of a relationship must be an indivisible data item （ There are no more tables in the table ）
  • The advantages and disadvantages of the relational model ：
    One advantage ：① The relational model domain format model is different , It is based on strict mathematical concepts ;
    ② Single concept of relationship model ;
    ③ Access path of relational model is transparent to users , So it has higher data independence 、 Better security , It also simplifies the work of programmers and database development ;

• Object oriented data model

• Object relational data model

• Semi structured data model

3 The structure of the database system

Database system schema concept

• There is... In the data model ” type “ and “ value ” The concept of .
• The pattern is relatively stable , And the examples are relatively variable ;

Three level pattern structure of database system

• 1, Pattern （schema）： Patterns are also called logical patterns , It is a description of the logical structure and characteristics of all data in the database , Is a common data view for all users .
• 2 External mode （external schema）： Also known as sub mode or user mode , It is the logical structure and feature description of local data that database users can see and use , It's the database user's data view , Is a logical representation of data related to an application .
• 3 Internal mode （internal schema）： Internal mode is also called storage mode （storage schema）, A database has only one internal schema . It is a description of the physical structure and storage of data , It's how the data is organized inside the database .

Database secondary image function and data independence （ One sentence summary : Adjacent levels have an image ）

• The database management system provides two layers of images between the three-level modes ： External mode / Mode image and mode / Internal mode image . Ensure that the data in the database system can have high logical independence and physical independence .

• 1 External mode / Pattern image ——> Logical independence

  • Modified the table structure ( It's a conceptual model ) You only need to modify the image between the external schema and the conceptual schema ( Change the external mode , Such as the definition of view ), Without modifying the user program .

• 2 Pattern / Internal mode image ——> Physical independence

  • Modified the physical storage method of data ( For example, add a secret ), Table structure ( It's a conceptual model ) Can remain the same .

Composition of data system

• 1 Hardware platform and database
  2 Software
  3 personnel

summary

The conceptual model is briefly introduced 、 The three elements and three main data models that make up the data model （ hierarchical model 、 Mesh model 、 relational model ）, The relationship model will be introduced in detail later .