Constraint for 2 foreign keys to have same secondary column value

Question

Suppose I have a table such as Users that has a id and a company_id I want to make a 2nd table that has pairs of user id's ex: (primary_key_id, user1_id, user2_id).

The user id's must be in the same company for these pairs of users.

How can I add a constraint to my database to require this?

Answer 1

You can create a unique constraint across company_id and id of the User table, then create two foreign keys that reference them.

Syntax for MySQL

CREATE TABLE `User` (
  id int PRIMARY KEY,
  company_id int,
  UNIQUE KEY (company_id, id)
);

CREATE TABLE UserPair (
  company_id int,
  user1_id int,
  user2_id int,
  PRIMARY KEY (company_id, user1_id, user2_id),
  FOREIGN KEY (company_id, user1_id) REFERENCES `User` (company_id, id),
  FOREIGN KEY (company_id, user2_id) REFERENCES `User` (company_id, id)
);

INSERT `User` (id, company_id) VALUES
(1,1),
(2,1),
(3,2);

INSERT UserPair (company_id, user1_id, user2_id) VALUES(1,1,2);

INSERT UserPair (company_id, user1_id, user2_id) VALUES(1,1,3);
-- Cannot add or update a child row: a foreign key constraint fails

db<>fiddle (for SQL Server, which is slightly different syntax)

The constraint fails because there is only a single company_id which each foreign key uses, but one of the corresponding parent rows has a different company_id.