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Introduction to database system - Chapter 1 introduction [conceptual model, hierarchical model and three-level mode (external mode, mode, internal mode)]
2022-07-07 13:38:00 【kokotao】
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 .
- 1 There is only one node without parents , This node is called the root node ;
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 .
- Data structure of mesh model :
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 ;
- Some terms of relational model :
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 .
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