制作Mysql数据库（外键）

Question

I have a task where I need to make a Database like social networks. I need to have 3 tables users, friendships, and status.

My problem is that I need to have a relationship with table friendships and table users.

I tried to do the task but I just don't get it. Also table status needs to have a relationship with table users (user_id ). I only know that table friendship needs to have 2 foreign keys.

Here is also an EER diagram of what is I tried to do.

Answer 1

You want two foreign keys on two different columns of friendships pointing to users :

create table friendships (
    request_sender   int not null,
    request_receiver int not null,
    ...
    constraint request_sender_users_fk  
        foreign key (request_sender)
        references users(user_id),
    constraint request_sender_receiver_fk  
        foreign key (request_receiver)
        references users(user_id)
);

Note: I don't think that it makes sense to have three different columns to store the status of the "friendship request", as in your ERD: friendship_accepted , friendship_declined and friendship_on_hold . Presumably a relation can only have one status at a time, so a single column should be good enough, with en enum or the-like.