Object relationship diagram

Interpretation of the object relationship diagram: A Brief E-R diagram refers to the three basic concepts of the object, relationship, attribute to summarize the basic structure of data, so as to describe the concept of static data structure. A E-R diagram is an entity-contact diagram that provides methods to represent the entity, attributes, and associations to describe the conceptual models of the real world. The most common application of E-R model is in the analysis stage of database design.

Interpretation of the object relationship diagram: A Brief E-R diagram refers to the three basic concepts of the object, relationship, attribute to summarize the basic structure of data, so as to describe the concept of static data structure. A E-R diagram is an entity-contact diagram that provides methods to represent the entity, attributes, and associations to describe the conceptual models of the real world. The most common application of E-R model is in the analysis stage of database design.

Explanation

Entity Relationship Diagram: A simplified E-R diagram refers to the basic structure of data summarized by three basic concepts: entity, relation, and attribute, so as to describe the concept pattern of static data structure. A E-R diagram is an entity-contact diagram that provides methods to represent the entity, attributes, and associations to describe the conceptual models of the real world. The most common application of E-R model is in the analysis stage of database design, that is, the communication tools and bridges between database designers and database users. The role of the E-R model is to construct a conceptual data model, and the Conceptual Data Model is the representation of the database structure, and this representation is not related to the database management system and data model.

Composition

Entity ):

It is the most basic concept of a relational database, used to describe objects in the real world.

It is represented by a rectangle, and the entity name is indicated in the rectangle. For example, student Zhang Sanfeng and student Li xunhuan are all entities.

Objects can be subdivided:

Strong entity: An entity that does not need to be attached to other entities. It is represented by a single rectangle.

Weak entity: refers to the entity that needs to be attached to other entities. It is represented by a bilateral rectangle.

Attribute ):

Used to describe the nature of an object.

It is represented by an elliptical shape and connected to the corresponding entity with an undirected edge. For example, the student's name, student ID, and gender are attributes.

Attributes can be divided:

Simple attribute: it cannot be further cut into other meaningful units.

Composite attribute: indicates that this attribute can be further cut in the future. For example, the address attribute is composed of city, county, township, specific address, and other attributes.

Derivative attributes ):

Contact (Relationship ):

Represents the relationship between an object and another object.

It is represented by a diamond. The contact name is specified in the diamond box and connected with the relevant entities with undirected edges respectively. At the same time, the contact type (1: 1, 1: 1: n or m: n) refers to three existing relationships (one-to-one, one-to-many, multiple-to-many ). For example, there is a relationship between the teacher's teaching and the student's course selection.

One-to-one relationship:

One-to-many or multiple-to-one relationships:

Many-to-many relationship:

The specific entity is as follows: