[database] basic theory

One 、 database

database (Database,DB) It's stored in the computer for a long time 、 organized 、 Shareable 、 A collection of related data under unified management . Literally , A database is a warehouse for storing data .

Two 、 Database management system (DBMS)

DBMS The function of : Database definition function ; Data access function ; Database operation management function ; Establishment and maintenance of database ; Data communication function .

DBMS The composition of : Language compiler handler ; System operation control program ; System establishment 、 Maintenance procedures ; The data dictionary .

3、 ... and 、 Database system (DBS) The composition of

data : Data is the working object of database system , They are necessary information for management and decision-making in a specific application environment .

user : User means storage 、 People who maintain and retrieve data in a database . The database system mainly includes 3 Class user : End user 、 Application programmers and database administrators .

Hardware : Hardware refers to storing database and running database management system DBMS Hardware resources .

Software : Software is responsible for database access 、 Software system for maintenance and management , It's usually called a database management system （DBMS）.

Four 、 Characteristics of database system

1. Low data redundancy 、 High sharing .

2. Improved data independence .

3. It has a unified data control function .

5、 ... and 、 Database application system

Database application system (Database ApplicationSystem,DBAS), It means in DBMS On the basis of , A user oriented system developed for a practical problem .

6、 ... and 、 The system architecture of the database

1. Internal architecture : The internal architecture of the database system is a three-level pattern structure , They are patterns 、 External mode and internal mode .

2. External architecture : The external architecture mainly includes centralized architecture 、 File server structure and client / Server structure .

7、 ... and 、 conceptual model

1. Entity : yes Real world Can be distinguished from other objects Of “ thing " Or object .

2. Entity set : Have the same type and share the same properties ( attribute ) Of Set of entities .

3. attribute : Entities are represented by a set of attributes ; Attributes are owned by each member in the entity set Descriptive nature .

4. keyword : An attribute or combination of attributes of an entity , Its value Can uniquely identify an entity , It's called the keyword , Also called code .

5. Domain : Each attribute has a Valueable set , The domain called this property , Or the value set of the property .

6. contact : The connection between two entities can be divided into one-to-one connection (1:1)、 One to many connection (1:n) And many to many connections (m:n).

    one-on-one (1:1)： for example Id card   citizens   One ID card corresponds to one citizen .
    One to many (1:n)： department <--> Department staff There are many employees in a department , An employee can only work in one department .
    Many to many (m:n)： Student <--> Course   A student has to study many courses , A course is studied by multiple students .

8、 ... and 、 Common data models

1. hierarchical model : Hierarchical data model is one of the earliest models used in database system , Its data structure is a directed tree . The hierarchy model has the following features : There is only one node without parents , This node is the root node . Other nodes have only one parent .

2. Mesh model : If you remove the two restrictions of the hierarchical model , That is, two or more nodes can have multiple parents , be “ Directed trees ” It becomes “ Directed graph ”.“ Directed graph ” Structure describes the mesh model . The mesh model has the following features : Can there be more than one node without parents . At least one node can have more than one parent .

3. relational model : relational model (Relational Model) Yes, it is Two dimensional table structure to represent entities and relationships between entities Data model of . The data structure of the relational model is a “ Two dimensional table frame ” Set of components , Every two-dimensional table can also be called a relation , So we can say , The relational model is “ Relationship framework ” Set of components .（ A relationship represents a two-dimensional table ）

Relational model is the most widely used data model , At present, most database management systems are relational , Such as MySQL、Sql server、Oracle It's all relational database management systems .

Common terms in relational models ：

① attribute / Field ： Column in table .

② Tuples ： The rows in the table .

③ Domain ： Value range of column .

④ Key words ( Primary key )： The attribute or attribute set that can uniquely represent a record .   

⑤ External keywords ( Foreign keys )： The relationship between reaction tables . If a column in one table is the primary key of another table , Then this column is the foreign key .