1. Four basic concepts of database system
Data: data
database: DataBase, referred to as DB
database management system: Database Management System, referred to as DBMS
database system: DataBase System, referred to as DBS
2. The characteristics of the database system
2.1, data structure
One of the main differences between database system and file system
2.2, high data sharing, low redundancy and easy to expand
2.3, high data independence
Data independence includes physical independence and logical independence.
Physical independence: the user's application and the physical data storage in the database are independent of each other.
Logical independence: the user's application and the logical structure of the database are independent of each other
2.4, the data is uniformly managed and controlled by the database management system
①Data security protection: protect data from data leakage and destruction caused by illegal use ②Data
integrity check: ensure the correctness, validity and compatibility of data
3. Data model
3.1, two data models
The first category: Conceptual model, also called information model, is mainly used for database design. The
second category: Logical model and physical model. Logical model mainly includes hierarchical model, mesh model, ***relational model***, etc., mainly For the realization of database management system, the physical model is the abstraction of the lowest level of data
3.2 Basic concepts of the information world
(1) Entity: Objects that exist objectively and are distinguished from each other become entities, such as a student and a teacher.
(2) Attribute: a certain characteristic of an entity is called an attribute, such as student number, name, date of birth
(3) Code: the attribute set that uniquely identifies the entity
(4) Entity type: Use the entity name and its attribute set to abstract and describe similar entities
(5) Connection: the connection between entities usually refers to the connection between different entity sets
3.3, the more important relational model in the logical model
Relationship: A relationship corresponds to a commonly-speaking table
Tuple: A row in the table is a tuple
Attribute: A column in the table is an attribute
code: A tuple
field can be uniquely determined : a group of the same data type It is worth collecting, for example, the domain (12~45 years old)
component of the attribute of the age of college students : an attribute value in the tuple.
Relation mode: generally expressed as: relation name (attribute 1, attribute 2, attribute 3,...) For example: student (Student ID, name, class, date of birth)
4. The three-level model structure of the database system
The three-level model of the database system means that the database system is composed of three levels: external mode, mode, and internal mode.
4.1 Mode
Mode is also called logical mode. It is the description of the logical structure and characteristics of all data in the database. It is the common data view for all users and the middle layer of the database system. There is only one mode for a database.
4.2 Outer mode
External mode is also called sub mode or user mode. It is a description of the logical structure and characteristics of local data that database users can see and use. It is a data view of database users.
4.3 Inner Mode
The internal mode is also called the storage mode. A database has only one internal mode, which is a description of the physical structure and storage mode of the data.
5. The secondary image function of the database and the data independence
