Basic concepts of database systems

the database system, DBS, contains databases (database, DB), database management systems (databases Management System,dbms), hardware, other software, and users.

Data Model:

First of all, what is a model?

Model can be divided into physical model and abstract model. For example, car models and aircraft models are physical models; the mathematical Model S = PI * r^2 is an abstract model that abstractly describes the number relationship between the area and radius of a circle.

The data model is an abstract model, and it is an abstraction of the characteristics of the data , and the data is the abstraction of things and their characteristics in the real world.

3 Types of data models:

1. The conceptual Data Model (conceptual), also known as the information model.

User-oriented model, regardless of the specific DBMS. As the language of communication between database designers and users, it serves for the purpose of database design application. can be transformed into a DBMS-supported structure (also known as a logical) data model, which is then implemented in the DBMS. The most common and well-known conceptual data model is the entity-contact (E-R) model, or E-R model.

2. The structural data Model (Structure) is also known as the logical data model.

The user-oriented and system-oriented data model serves the application realization of DBMS. Is the data model that the user sees from the database and is supported by the specific DBMS. Associated with a specific DBMS. Generally, a DBMS supports only one data model. The more mature and widely used structure data model has hierarchical model, net model, relational model, object-oriented model and object-oriented relation model, etc.

3. Physical Data Model (physical).

The organizational structure used to describe the data on physical storage media, related to the specific DBMS, and to the specific operating system and hardware. Each structure data model must have its corresponding physical data model when it is implemented.

Conceptual Data Model:

E-r model three elements: entity, attribute, contact.

Entities: Things that can be distinguished from one another in the real world.

Properties: Characteristics of a particular aspect of an entity.

Contact: A connection between entities.

entity type : abstract and characterize a homogeneous entity, known as entity type, with a collection of entity names and attribute names.

For example, a student (school number, name, gender, date of birth, time of entry) is a physical type.

entity set: a collection of several entity types is called an entity set.

For example, all students are an entity set.

Categories of Contact:

1. From different levels of contact:

A contact within an entity set: A connection between entities within an entity set.

Linkage between entity sets: a contact between an entity in one entity set and an entity in another entity set.

2. From the standpoint of the manifestation of the link:

Existence of sexual relations: such as the existence of teachers in schools.

Functional Contact: If the teacher teaches.

Event Contact: such as a student borrowing books.

3. In terms of the "meta-number" of linkages between entities:

One-on-one contact (1:1): if for each entity a in entity set a, the entity set B has at most an entity B associated with it, and vice versa. Recorded as 1:1.

One -to-many connections (1:N): for each entity a in entity set a, there are n entities associated with entity set B, and for each entity B in entity set B, there is at most one entity in entity set a associated with it, which is said to have 1-to-many connections to entity set A and entity set B. Remember as 1:n.

Many-to-many connections (M:N): If there are N entities in entity set B associated with each entity in an entity set a, and for each entity in entity set B, entity set A is associated with a m entity, which is called the entity set A and entity set B have many-to-many connections, which are recorded as m:n.

Unary Contact: A contact within an entity.

Two-element contact: Two relationships between entities.

Multi-Contact: 3 or more than 3 entities.

e-r diagram : A graphical representation of the E-R model.

Rectangle represents an Entity

Ellipse Representation Property

Diamond indicates contact

Structure Data Model:

In general, a structured data model is a set of strictly defined sets of concepts, consisting primarily of 3 parts of data structures, operations, and integrity constraints , often referred to as data Model 3 features.

Relational Model:

A relational model is a structural data model that represents a set of entities using a two-dimensional table. Tables are made up of rows and columns.