TSQL Problem - Link Table

I am trying to do quite a straight forward TSQL link table but I am falling at the first hurdle, basically I want a two way link.

For example ....

If Contact 1 adds contact 2, then contact 2 is connected to contact 1 (and vice versa) the idea being that the query recognises they are paired up.

Table Example

RelatedDataID  UniqueID   Related_UniqueID
-------------------------------------------
1              AA          BB
2              CC          DD
3

Users Table

UserID  UniqueID   Username
----------------
1        AA       Bob
2        BB       Fred
3        CC       Charlie
4        DD       Billy

So basically when I run a query With UniqueID "AA" it will return

Username     RelatedID
------------------------
Bob          1
Fred         1
Charlie      0
Billy        0

But also when I run it on UniqueID "CC" it should return

Username   RelatedID
---------------------
Bob        0
Fred       0
Charlie    2
Billy      2

Does anyone know how I can achieve this? My current stored procedure seems to only bring back ones that are linked, and not the ones that are not linked. I need it to bring back a full list of all users, but to have the RelatedID come back as either a 0 or the RelatedDataID.

The UniqueID's are GUIDs.

Here my TSQL statement.

ALTER PROCEDURE sp_Test
 @CompanyID int,
 @UniqueID varchar(36),
 @PersonTypeID int
AS
BEGIN
 SET NOCOUNT ON;

 SELECT
  First_Name + ' ' + Last_Name AS Full_Name,
  ISNULL(RelatedDataID,0) AS RelatedDataID
 FROM
  Users
 LEFT JOIN
  Related_Data
 ON
  Users.UniqueID = Related_Data.UniqueID
 WHERE
  Users.PersonTypeID = @PersonTypeID
 AND
  Users.Deleted = '0'
 AND
  ((ISNULL(Related_Data.UniqueID,'') = '') OR (Related_Data.UniqueID = @UniqueID OR Related_Data.Related_UniqueID = @UniqueID))

(UPDATE) Given that you mentioned in comments that the UniqueId columns are guids and you want a zero to return when no relations, we have to case the guids to varchar. It is also not clear what the expected output should be if the a given user has multiple relations. This query will choose a relation on User.UniqueID = RelatedData.UniqueId over a relation on User.UniqueId = RelatedData.Related_UniqueId but will only return one row per user.

Select U.username
    , U.FirstName + ' ' + U.LastName As Full_Name
, Case
    When RD.Related_UniqueID Is Not Null Then RD.RelatedDataId
    When RD1.UniqueId Is Not Null Then RD1.RelatedDataId
    Else 0
    End As RelatedID
From Users As U
    Left Join Related_Data As RD
        On RD.UniqueID = U.UniqueId
            And (
                RD.Related_UniqueID = @UniqueId
                Or RD.UniqueId = @UniqueId
                )
    Left Join Related_Data As RD1
        On RD1.Related_UniqueID = U.UniqueId
            And RD1.UniqueID = @UniqueId
Where U.Deleted = '0'
    And U.PersonTypeId = @PersonTypeId