E-R模型用E-R圖表示。實體是用戶工作環境中所涉及的事務，屬性是對實體特征的描述。

該系統的總體ER圖中總共包含6個實體，分别是用戶、訂單、書籍、管理員、類别。

The E-R model is represented by an E-R diagram. The entity is the transaction involved in the user's work environment, and the attribute is the description of the entity's characteristics.

The overall ER diagram of the system contains a total of 6 entities, namely users, orders, books, administrators, and categories.

（1）若實體間的聯系是1：1

兩個實體類型轉換成的兩個關系模式中任意一個關系模式的屬性中加入另一個關系模式的鍵和聯系類型的屬性。

（2）若實體間的聯系是1：N

在N端實體類型轉換成的關系模式中加入1端實體類型轉換成的關系模式的鍵和聯系類型的屬性。

（3）若實體間的聯系是N：M

将聯系轉換為實體，然後在該實體上加上另外兩個實體的主鍵，作為聯系實體的主鍵，然後再加上該聯系自身帶的屬性。

(1) If the connection between entities is 1:1

The attributes of any one of the two relational schemas converted by the two entity types are added with the key of the other relational schema and the attributes of the connection type.

(2) If the connection between entities is 1:N

Add the key and connection type attributes of the relational model converted from the 1-end entity type to the relational model converted from the N-end entity type.

(3) If the connection between entities is N: M

Convert the contact into an entity, and then add the primary keys of the other two entities to the entity as the primary key of the contact entity, and then add the attributes of the contact itself.

根據以上轉換規則可得校園二手書交易系統的數據庫設計邏輯結構為:

According to the above conversion rules, the logical structure of the database design of the campus second-hand book trading system can be obtained as:

用戶（用戶名，密碼）

管理員（用戶名，密碼)

書籍（書籍ID，價格）

訂單（書籍ID ，購買人，價格，數量）

User (user name, password)

Administrator (user name, password)

Books (book ID, price)

Order (book ID, purchaser, price, quantity)

素材來源：百度、知乎

翻譯來源：谷歌翻譯‍