What Are Associative Entities?

ER model , which is called Entity-Relation Model , Entity-Relation Model or Entity-Relationship Model Diagram (ERD) (English: Entity-relationship model) was invented by Chinese American computer scientist Chen Pinshan. It is a data model or a high-level description of a conceptual data model. Pattern illustration.

Entity

Represented by a rectangle, and the entity name is written in a rectangular box; for example, a student

Principles of transformation into a relational model

1. An entity is transformed into a relational model. The entity attribute is the attribute of the relationship, and the code of the entity is the code of the relationship.

2. A 1: 1 relationship can be transformed into an independent relationship model, or it can be merged with the corresponding relationship at either end.

3. A 1: n connection can be transformed into an independent relationship model, or it can be merged with any n-terminal corresponding relationship.

4. An m: n connection is transformed into an independent relationship model.

5. Multiple connections between three or more entities can be transformed into a relationship model.

6. Relationship modes with the same code can be merged.

A weak entity is an entity that cannot be uniquely identified by its own attributes, so it must use both its own attribute and the primary key of the entity to which it is associated as its primary key. The set of weak entities is indicated as a thick rectangle (solid), connected by a thick arrow to a thick diamond (connection).

Sometimes two entities are more specialized subtypes of a more general entity type. For example, programmers and marketers are both types of employees in software companies. To indicate this association, draw a triangle with "ISA" in it.

The model of entity contact can be classified into BERM (binary entity association model) and GERM (general entity contact model) according to whether only binary contact is allowed. A binary connection is a connection between two entities. Therefore, connections between three or more entities are also allowed in GERM.

• entity

• Relational model

• Unified modeling language

• Data structure diagram