What is an E-R diagram?

E-R diagram, also called entity-relationship diagram, is used to show the relationship between entity sets. It provides methods for representing entity types, attributes, and relationships; a conceptual model used to describe the real world. An ER model is a design or blueprint of a database that can later be implemented as a database.

In an E-R diagram, an entity set is a group of similar entities (data objects in the data model) that can have attributes. In a database system, an entity is a table in the database or an attribute in a table, so by showing the relationship between tables and their attributes, ER diagram shows the complete logical structure of the database. [Video tutorial recommendation: MySQL tutorial]

Composition of E-R diagram:

1. Entity

Entity represents the data object or component in the data model. For example: people, students, and music can be used as a data object; use a rectangle in the ER diagram To represent.

There is a special kind of entity in the data model: a weak entity, which cannot be uniquely identified by its own attributes and relies on relationships with other entities. In ER diagrams, weak entities are represented by double rectangles.

Note: Each entity has its own entity members or entity objects, but these members or objects do not need to appear in the ER diagram.

2. Attributes

Attributes represent the attributes of an entity, for example: a person’s name, age, address and other attributes; in the ER diagram Represented by an ellipse, there are four types of attributes:

1), key attributes: can uniquely identify entities in the entity set

2), composite attributes: belong to a combination of other attributes Attributes

As can be seen from the above figure: "Student ID" is the key attribute; "Address" is a composite attribute, because "Address" is also composed of other attributes, such as: Province, city, district/county.

3) Multi-valued attributes: Attributes that can contain multiple values ​​are called multi-valued attributes and are represented by double ellipses.

4), Derived attributes: The value is dynamic and derived from another attribute, represented by a dotted ellipse.

3. Relationship

Relationship is used to express the connection between entities. It is represented by a diamond in the ER diagram. There are four types of relationships. ：

1), one-to-one

2), one-to-many

3), many-to-one

4), many-to-many

E-R diagram example:

Summary: The above is the The entire content of this article, I hope it will be helpful to everyone's study.

The above is the detailed content of What is an E-R diagram?. For more information, please follow other related articles on the PHP Chinese website!