title: Foreign key reference to a specific subtype
body: <p>This question piggybacks off of <a href="https://stackoverflow.com/questions/7844460/foreign-key-to-multiple-tables">Foreign Key to multiple tables</a>
  but I thought that my elaboration could use its own subject.</p>
  
  <p>Say I have the following schema (adapted from <a href="https://stackoverflow.com/users/3769/nathan-skerl">@Nathan Skerl's</a> answer in the above link):</p>
  
  <pre><code>    create table dbo.PartyType
  (   
      PartyTypeId tinyint primary key,
      PartyTypeName varchar(10)
  )
  
  insert into dbo.PartyType
      values(1, 'User'), (2, 'Group');
  
  
  create table dbo.Party
  (
      PartyId int identity(1,1) primary key,
      PartyTypeid tinyint references dbo.PartyType(PartyTypeId),
      unique (PartyId, PartyTypeId)
  )
  
  CREATE TABLE dbo.[Group]
  (
      ID int NOT NULL,
      Name varchar(50) NOT NULL,
      PartyTypeId as cast(2 as tinyint) persisted,
      foreign key (ID, PartyTypeId) references Party(PartyId, PartyTypeID)
  )  
  
  CREATE TABLE dbo.[User]
  (
      ID int NOT NULL,
      Name varchar(50) NOT NULL,
      PartyTypeId as cast(1 as tinyint) persisted,
      foreign key (ID, PartyTypeId) references Party(PartyID, PartyTypeID)
  )
  
  CREATE TABLE dbo.Ticket
  (
      ID int NOT NULL,
      [Owner] int NOT NULL references dbo.Party(PartyId),
      [Subject] varchar(50) NULL
  )
  </code></pre>
  
  <p>And say I had another relation, say 'Stuff', where 'Users' could possess many items of 'Stuff', but it did not make sense for 'Groups' to have 'Stuff'. Could I have a Foreign Key in 'Stuff' reference just 'User'? </p>
  
  <p>If it is possible, then how would one go about doing so? Or do I have to do all of my foreign keys of that sort directly through 'Party'?
  I got an error (<em>There are no primary or candidate keys in the referenced table 'dbo.Users' that match the referencing column 'ID' in the foreign key</em>) when I tried myself.</p>
  
  <p>Thanks in advance!</p>