Cross table constraints in PostgreSQL

Question

Using PostgreSQL 9.2.4, I have a table users with a 1:many relation to the table user_roles. The users table stores both employees and

Answer 1

First, you can solve this using a trigger.

But, I think you can solve this using constraints, with just a little weirdness:

create table UserRoles (
    UserRoleId int not null primary key,
    . . .
    NeedsEmployeeNumber boolean not null,
    . . .
);

create table Users (
    . . .
    UserRoleId int,
    NeedsEmployeeNumber boolean,
    EmployeeNumber,
    foreign key (UserRoleId, NeedsEmployeeNumber) references UserRoles(UserRoleId, NeedsEmployeeNumber),
    check ((NeedsEmployeeNumber and EmployeeNumber is not null) or
           (not NeedsEmployeeNumber and EmployeeNumber is null)
          )
);

This should work, but it is an awkward solution:

• When you add a role to an employee, you need to add the flag along with the role.
• If a role is updated to change the flag, then this needs to be propagated to existing records -- and the propagation cannot be automatic because you also need to potentially set EmployeeNumber.