困在mysql数据库中的多对多关系

Question

I am doing a simple Shopping application as my project. I am stuck to maintain a many-to-many relationship.

I have to store all the details of a user transaction in some tables with relationship. Here are some important columns.

User_Id -- Foreign key from user table
Associated_Product_Id -- Foreign key from product table
Associated_Product_Quantity -- Not Null

For more simplicity

user1         product1         5 pieces
user1         product2         2 pieces
user1         product3         1 piece
user2         product1         2 pieces
user2         product3         3 pieces

Here one user can buy many products and one product can be bought by many users. So I am totally confused, how to manage this table. I mean which one is primary key here.

It doesn't matter in how many tables I will manages these information. But I need to manage thes information. Can anybody give a rough idea on managing this. So that I could work on that direction.

Thanks.

Answer 1

For the transaction you just need 1 table - something like you just posted.

If you need to know what products a certain user bought, you query this table for User_Id , and if you need to know by what users a certain product was bought, you query the table for Associated_Product_Id . (I assume that you already have your table for producs and users, since you are referencing the IDs here.)

You should of course add a column Transaction_Id or something similar, to uniquely identify a transaction.

Answer 2

Since you are doing a shopping application as a project, you may want to consider adding a fourth table into your schema, to hold Orders.

Your tables might then look something like:

Users
-----
UserID (PK)
... (Other user details)

Product
-------
ProductID (PK)
... (Other product details)

Order
-----
OrderID (PK)
UserID (FK)
OrderDate

OrderLine
---------
OrderID (PK)
ProductID (PK)
OrderQuantity

This would then enable you to see how many units of a product a user bought on any given date, rather than just how many units that user has ever bought.

Answer 3

You'll want three tables:

1) User Table 2) Product Table 3) Transaction Table

User 1:m Transaction Transaction 1:m Product

Transaction will need keys to link both User & Product

Answer 4

MySQL doesn't let you use many-to-many relationships in only two tables. You have to create a third table for the two tables you want to relate to.

The third table will simply contain foreign keys for the products and users (plus an ID for each transaction).

A simple example woulc be:

CREATE TABLE Product_User (
    trans_id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    product_id INT NOT NULL, 
    user_id INT NOT NULL, 
    FOREIGN KEY (product_id) REFERENCES Product_table (productid), 
    FOREIGN KEY (user_id) REFERENCES User_table (userid)
);

With the corresponding names for the tables and the id fields.