SQL 如何正确建立关系？

Question

I'm having a lot of trouble understanding the sql tables relationships, I have a big mess in my brain right now, so I wanna ask a few questions.

Context: I'm doing database for an basic app that allows users to have emergencies and register them.

So, I have my user table and emergency table, and I want to relationship them, what should I ask to figure it out the relationship?

How many emergencies can an user have? at the same time? only one. In total? many How many users can have an emergency? (at the same time? many users. The same single emergency? only one.

So what would be the relationship? one-to-one: Only one user can have the same emergency at the same time, and that one emergency can have only that one user having it.

one-to-many: One user can have many emergencies in his lifetime, and one emergency can happen at the same time by that user

many-to-many: Many users can have emergencies at the same time, and many emergecies can be happening at the same time to users

Which one of those should be my thinking? or neither of them? Should I be thinking in plural or singular?

And where the foreign key should go? I can't understand that yet.

I'm only doing databases with workbench EER models, maybe that's why I'm so confused.

Thank you for your time.

Answer 1

This is admittedly a pretty tricky set of requirements, so it's understandable that you were having trouble modeling it.

Here's a proposed table design that I think would support the cases you described.

CREATE TABLE users (
 user_id INT PRIMARY KEY
 current_emergency_id INT NULL
);

CREATE TABLE emergencies (
 emergency_id INT PRIMARY KEY,
 user_id INT NOT NULL,
 KEY (user_id, emergency_id),
 FOREIGN KEY (user_id) REFERENCES users(user_id),
);

ALTER TABLE users ADD FOREIGN KEY (user_id, current_emergency_id)
 REFERENCES emergencies (user_id, emergency_id);

This is a circular reference, there are foreign keys in both tables referencing the other. You can't define a foreign key before the table it references exists, so at least one of the foreign keys must be added after both tables have been created.

This design allows a user to have many emergencies, because one or more rows in emergencies may reference the same user_id value.

It allows a user to have just one current emergency, because the user.current_emergency_id can only have one value for a given user at a time. This column is nullable, so it is also allowed for a given user to have no current emergency, though they have had some in the past. Also the compound key is used so that the foreign key must reference the pair of values. We don't want a user's current emergency to reference an emergency that is not assigned to that user.

This supports many emergencies existing concurrently, as long as they are assigned to different users. Since each user can reference at most one emergency as its current emergency, and each user must reference an emergency assigned to itself, then the set of current emergencies must all be assigned to distinct users.