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

New Answer:

This( SQL Sub queries in check constraint ) seems to answer your question, and the language is still in the 9.4 documentation( http://www.postgresql.org/docs/9.4/interactive/sql-createtable.html ).

Old Answer:

SELECT 
  User.*
  , UserRole1.*
FROM
 User
  LEFT JOIN UserRole UserRole1 
    ON User.id = UserRole1.UserId 
     AND (
      (
       User.employee_number IS NOT NULL AND UserRole1.role_name IN (enumerate employee role names here)
      ) 
     OR 
      (User.employee_number IS NULL)
     )

The above query selects all fields from User and all fields from UserRole(aliased as UserRole1). I assumed that the key field between the two fields is known as User.id and UserRole1.UserId, please change these to whatever the real values are.

In the JOIN part of the query there is an OR that on the left side requires an employee number not be NULL in the user table and that UserRole1.role_name be in a list that you must supply to the IN () operator.

The right part of the JOIN is the opposite, it requires that User.employee_number be NULL(this should be your non-employee set).

If you require a more exact solution then please provide more details on your table structures and what roles must be selected for employees.