In database technology, the four main data models are: 1. Hierarchical data model, which uses a "tree" structure to represent the association between entity sets; 2. Network data model; 3. Relational data Model organizes data in the form of record groups or data tables to facilitate storage and transformation using the relationships between various entities and attributes; 4. Object-oriented data model.
The operating environment of this tutorial: Windows 10 system, Dell G3 computer.
In database technology, the four main data models are: hierarchical data model, network data model, relational data model and object-oriented data model.
Hierarchical data model
The hierarchical data model is the earliest model used in database systems. It uses a "tree" structure to represent the association between entity sets. , where the entity set (represented by a rectangular box) is a node, and the connection between nodes in the tree represents the association between them.
Function:
The data model that uses a hierarchical structure to represent entity types and relationships between entities is called a hierarchical data model (hierarchical datamodel). The relationship between records is realized through pointers, and the query efficiency is high. .
Representation method:
The representation method of the hierarchical model is: the nodes of the tree represent the entity set (record type), and the connection between the nodes represents the connection between the two entity sets. Relationship, this relationship can only be "1-M". Usually, the entity set representing 1 is placed at the top, called the parent node, and the entity set representing M is placed at the bottom, called the child node.
There are two implementation methods for physical storage of the hierarchical model:
Sequential method: all records are stored adjacently in hierarchical order, that is, adjacent through the location of the physical space to achieve hierarchical order.
Pointer method: Each record is not stored in hierarchical order, but uses pointers to link them in hierarchical order.
Network data model
The network model uses connection instructions or pointers to determine the network connection relationship between data. It has multiple How to organize multiple types of data.
The network model is a database model that can flexibly describe things and the relationships between them. It was first invented by Charles Bachman in the United States.
The data structure of the mesh model mainly has the following two characteristics:
(1) Allows more than one node to have no parents.
(2) At least one node can have more than one parent.
Each node in the network model represents a record type (entity). Each record type can contain several fields (attributes of the entity). The connections between nodes represent the relationships between record types (entities). father-son relationship.
It can be seen from the definition that the connection between the child node and the parent node in the hierarchical model is unique, but in the network model this connection may not be unique. Therefore, in a network model, each link is named and the parent and child records associated with that link are indicated.
Relational Data Model
The relational model organizes data in the form of record groups or data tables to facilitate storage and use of relationships between various entities and attributes. Transformation, without hierarchies or pointers, is a very effective data organization method for establishing relationships between spatial data and attribute data.
The relational model consists of three parts: relational data structure, relational operation set and relational integrity constraints.
The relational model allows the definition of three types of integrity constraints; entity integrity, referential integrity and user-defined integrity. Entity integrity and referential integrity are integrity constraints that the relational model must satisfy, reflecting semantic constraints in specific fields.
Entity integrity rules: If attribute A is the main attribute of basic relationship R, attribute A cannot take a null value. Entity integrity rules stipulate that all primary attributes of a basic relationship cannot have null values, not just the entire primary key.
(1) Entity integrity rules are for basic relationships. A basic table usually corresponds to an entity set in the real world. For example, the student relationship corresponds to the collection of students.
(2) Entities in the real world are distinguishable, that is, they have some uniqueness mark.
(3) Correspondingly, the primary key is used as the uniqueness mark in the relational model.
(4) The attributes in the main code, that is, the main attributes, cannot take a null value. The so-called null value is an "unknown" or "meaningless" value. If the main attribute takes a null value, it means that there is an unidentifiable entity, that is, there is an indistinguishable entity. This contradicts point (2), so the rule becomes entity integrity.
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