Learn to draw Er graph in an article

What is? er chart ？

E-R Graph is also called entity relation graph , Is an entity that provides , Properties and contact methods , A conceptual model used to describe the real world . In layman's terms , When we understand the needs of the actual problem , There needs to be a way to express this requirement , The conceptual model is used to describe such requirements .

such as ： Departments and majors in universities , And students' noumenon , Follow the following picture roughly er chart

er The basic elements in the diagram

Let's use the picture above to illustrate ：

• Entity ： Things that exist in practical problems and can be identified with each other

Like the one above ： departments , major , Male students , Girl friend

• attribute ： A property of an entity , We call them attributes

such as ： The above picture can be used by departments ： Department number , name , Attribute description of mailbox

A major can use a major code , Professional name Description

Male students can use their student ID , Name Description

• Entity set ： A collection of entities with the same attributes ,（211……13, Ming Di ） It is an entity in the student entity set

• key ： Attributes that uniquely identify entities are called keys , Keys are also attributes of entities , The attribute value as a key must be unique and cannot be empty . For example, student ID. 211……13 It can be called a key

• 1 Yes 1,1 For more than , Many to many ,3 Kind of

draw er Graph specification

• Rectangle box ： Represents an entity , Write the entity name in the box

• Diamond frame ： It means to contact , Write the contact name in the box

• Oval box ： Represents the property , Write the attribute name in the box

• attachment ： Entities and attributes , Entity and connection , Connection and attribute
  Use straight lines
  Connect ,

• In the process of connection, the type of connection shall be marked on the line ：

• 1 Yes 1： Contact each direction 1

• 1 For more than ：1 The party who wrote 1, More than one party to write n

• Many to many ： Write in each direction n,m

example

Through the above explanation , Let's do an exercise .

Library management system should be no stranger , A brief analysis of , The discovery database mainly provides the following services ：

• You can query the varieties of existing books in the library at any time 、 Quantity and storage location . All kinds of books can be uniquely identified by the book number .

• You can check the borrowing and returning of books at any time , Including the borrower's unit 、 full name 、 Library card number 、 Borrowing date and return date .

• Appointment ： Anyone can borrow a variety of books , Any kind of book can be borrowed by more than one person , The library card number is unique .

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