我的主键和外键是什么？

Question

I am newbie to Database: what will be my Primary and Foreign keys for the following database: user buys items and rates them by giving ratings(1,2,3,4,5)
user(id, name)
item(id, description, category, price) buys(user, item, price) rates(user, item, rating)

Answer 1

Primary key

• id in User table
• id in item table

Foreign key

• user_id (referring User table PK) and item_id (referring item table PK) in buys table

• user_id (referring User table PK) and item_id (referring item table PK) rates table

Answer 2

In the given situation acc. to me this is what you have to take following columns as:

Primary Key:

Id in User table
Id in Item Table

Foreign Key:

User in Buys Table Of ID in User Table
Item in Buys Table Of ID in Item Table
User in Rates Table Of ID in User Table
Item in Rates Table Of ID in Item Table

Answer 3

PK = primary key FK = foreign key

user(PK userid, name)

item(PK itemid, description, category, price) 

buys(FK userid, FK itemid, price) 

rates(FK userid, FK itemid, rating)

buys needs a price only if the price can change.

The user and item tables need a primary key so they can be referenced by the buys and rates tables. I used different names than id so it is clear which columns to join over when doing a select with a join operation.