E-R draw clear content

About E-R chart , The relationship between data tables should be clear , Be clear , What is the function of each table , What is the relationship between tables

One 、 What is? E-R chart ？
E-R Graphs are also called entities - Contact diagram (Entity Relationship Diagram), Provides representation entity types 、 Properties and contact methods , A conceptual model used to describe the real world

Two 、 Part of the ：
Entity ： It is generally believed , What can be objectively distinguished from each other is the entity , Entities can be concrete people and things , It can also be abstract concepts and connections . Use a rectangle to represent
attribute ： A property of an entity , An entity can be characterized by several attributes . Attributes cannot be detached from entities , Attributes are relative to entities . For the main attribute name , Underline its name

contact ： It also has a relationship , The information world reflects the relationship within or between entities . The relationship within an entity usually refers to the relationship between the attributes that make up the entity ; The relationship between entities usually refers to the relationship between different sets of entities

3、 ... and 、E-R The standard of the diagram should be very clear , The primary key and other keys in each table should be clear distinguish , Make rational use of the storage capacity of the database , Improve the reading and saving speed of the database

Four 、 step ：
1、 Identify all entity sets ;

2、 Select the attributes that the entity set should contain
3、 Determine the relationship between entity sets

4、 Determine the primary key of the entity set , Underline the attributes to indicate the attribute combination of the primary key
5、 Determine the type of contact , When a diamond box representing a connection is connected to an entity set with a line , Note next to the line is 1 or n( many ） To indicate the type of connection

Acceptance criteria ：

1、 Express words accurately , The entity is a noun , Connection is a verb .

2、 Whether the graphics are used accurately , See above .

3、 The relationship between entities is correct , For example, the relationship between students and courses is many to many .

4、 Entity names and attributes correspond to table names and fields in the database . The entity is the name of the table in the database , Property is a field in the database .