Multiple Yet Mutually Exclusive Foreign Keys - Is This the Way to Go

Multiple yet mutually exclusive foreign keys - is this the way to go?

If we look into the model here, we will see the following:

1. A user is related to exactly one website
   
   
   • A company is related to exactly one website
   • A website is related to exactly one user or company

The third relation implies existence of a "user or company" entity whose PRIMARY KEY should be stored somewhere.

To store it you need to create a table that would store a PRIMARY KEY of a website owner entity. This table can also store attributes common for a user and a website.

Since it's a one-to-one relation, website attributes can be stored in this table too.

The attributes not shared by users and companies should be stored in the separate table.

To force the correct relationships, you need to make the PRIMARY KEY of the website composite with owner type as a part of it, and force the correct type in the child tables with a CHECK constraint:

CREATE TABLE website_owner (
    type INT NOT NULL,
    id INT NOT NULL,
    website_attributes,
    common_attributes,
    CHECK (type IN (1, 2)) -- 1 for user, 2 for company
    PRIMARY KEY (type, id)
)

CREATE TABLE user (
    type INT NOT NULL,
    id INT NOT NULL PRIMARY KEY,
    user_attributes,
    CHECK (type = 1),
    FOREIGN KEY (type, id) REFERENCES website_owner
)

CREATE TABLE company (
    type INT NOT NULL,
    id INT NOT NULL PRIMARY KEY,
    company_attributes,
    CHECK (type = 2),
    FOREIGN KEY (type, id) REFERENCES website_owner
)

Two mutually exclusive many-to-many relationships

I decided to settle on my last idea. Normalize sizes table, by creating intermediate productvariants table, which holds entities representing things customers buy.

orders table:

id int not null,
...

orders_productvariants table:

order_id int not null,
productvariant_id int not null,
...

productvariants table:

id int not null,
product_id int not null,
size_id int not null,
...

products table:

id int not null,
...

sizes table:

id int not null,
...

Additionally, I have a size with empty name, for items that have no size.

Can a linking table have two mutually exclusive columns?

Here's a thought to fix the problem without breaking much code. Create a newtable students and populate with the data from both the other tables and a flag field to indicate full or part time. Then create views that have the names of the full time students and part time students table. This means all selects will still work perfectly and all you will have to do is fix the insert and update code to go to the new table.

Or you could enforce your rules in a trigger.

Is there a way to restrict foreign keys references to another column putting custom rules on this references?

Yes, you can create a trigger before insert/update that will check whether exists an employee whose user_id matches the value of employer_id. If such a record exists, then throw an exception. See: https://www.postgresql.org/docs/9.1/plpgsql-trigger.html

Two foreign keys, one of them not NULL: How to solve this in SQL?

Your data model is fine. In most databases, you would also add a check constraint:

alter table `time` add constraint chk_time_project_special_work
    check (project is not null xor special_work is null);

However, MySQL does not support check constraints. You can implement the logic using a trigger, if you really like.

Join two tables through third one with two foreign keys

If you want to know the deposits in which a bank does not participate:

1- Add a related_name to deposit and bank:

class DepositProposal(models.Model):
    percent = models.FloatField()
    deposit = models.ForeignKey(Deposit, related_name = "proposals")
    bank = models.ForeignKey(Bank, related_name = "proposals")

2- Get the bank that you want to check:

bank = Bank.objects.first() #For example, the first bank

3- Get the queryset:

deposits = Deposit.objects.exclude(proposals__bank = bank)

EDIT:

If you want to know the banks that not participate on a specific deposit:

deposit = Deposit.objects.first() #For example, the first deposit
banks = Bank.objects.exclude(proposals__deposit = deposit)

Two foreign keys, only one can be not null at a time

Use a check constraint:

alter table t add constraint chk_table_fk1_fk2 on table
    check (fk1 is null or fk2 is null);

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