Question

For example, having the tables:

Companies (CompanyID)
Workers (WorkerID, CompanyID)
Users (WorkerID, Username)

With the correct relationships defined, how do I enforce the predication that a User’s Username is unique in the company where its worker is in?

Answer 1

You can use an indexed view for this

CREATE VIEW dbo.EnforceConstraint
WITH SCHEMABINDING
AS
SELECT CompanyID, Username
FROM dbo.Users u 
    JOIN dbo.Workers w ON w.WorkerID = u.WorkerID

GO

CREATE UNIQUE CLUSTERED INDEX ix ON dbo.EnforceConstraint(CompanyID, Username)

There are certain SET options that need to be on for modification of the base tables of indexed views but if you are on SQL Server 2005+ these are on by default.