declare @UserId varchar(20) = '1'
  declare @Email varchar(50)= 'blah1'

  -- set test src tables
  DECLARE @src TABLE
    (Id int,UserId nvarchar(20),Email nvarchar(50),PostCode int,Country nvarchar(50))

  -- inset test data
  INSERT @src SELECT *
    FROM (VALUES
    (1, '1', 'blah1', 111, 'au'),
    (2, '1', 'blah2', 111, 'au'),
    (3, '1', 'blah3', 111, 'au'),
    (4, '2', 'blah4', 111, 'au'),
    (5, '2', 'blah3', 111, 'nz'),
    (6, '4', 'blah4', 111, 'nz'),
    (7, '5', 'blah4', 111, 'nz'),
    (8, '5', 'blah6', 111, 'nz'),
    (9, '7', 'blah7', 111, 'nz'),
    (10, '8', 'blah8', 111, 'nz'),
    (11, '9', 'blah9', 111, 'nz'),
    (12, '10', 'blah10', 111, 'nz'),
    (13, '1', 'blah11', 111, 'nz')
    ) src (Id, UserId, Email, PostCode, Country)

  -- show the records, for debug purposes
  SELECT *FROM @src

  -- declare table variables
  DECLARE @Ids TABLE (Id int)
  DECLARE @Emails TABLE (Email varchar(50),Searched bit)
  DECLARE @UserIds TABLE (UserId varchar(20),Searched bit)
  DECLARE @Results TABLE (Id int,
    UserId varchar(20),
    Email varchar(50),
    PostCode int,
    Country nvarchar(50)
  )

  -- insert Initial UserId
  IF (@UserId IS NOT NULL)
    INSERT INTO @UserIds (UserId, Searched) VALUES (@UserId, 0);

  -- insert Initial Email
  IF (@Email IS NOT NULL)
    INSERT INTO Emails(Email, Searched) VALUES (t.Email, 0);

  -- while both variables have something in them, there are potentially more matches
  WHILE (@UserId IS NOT NULL OR @Email IS NOT NULL)
  BEGIN

    -- clear results
    DELETE FROM @Results

    -- Main search query
    INSERT INTO @Results SELECT Id,UserId,Email,PostCode,Country
      FROM @src
      WHERE (@UserId IS NOT NULL AND userId = @UserId)
      OR (@Email IS NOT NULL AND Email = @Email)

    -- if results are found merge
    IF (@@ROWCOUNT > 0)
    BEGIN

      -- merge new ids
      MERGE INTO @Ids i
        USING (SELECT Id FROM @Results) t ON t.Id = i.Id
        WHEN NOT MATCHED THEN
        INSERT (Id) VALUES (t.Id);

      -- merge new userIds
      MERGE INTO @UserIds u
        USING (SELECT UserId FROM @Results) t ON t.UserId = u.UserId
        WHEN NOT MATCHED THEN
        INSERT (UserId, Searched) VALUES (t.UserId, 0);

      -- merge new emails
      MERGE INTO @Emails u
        USING (SELECT Email FROM @Results) t ON t.Email = u.Email
        WHEN NOT MATCHED THEN
        INSERT (Email, Searched) VALUES (t.Email, 0);

    END

    -- mark variables as searched in thier respective tables
    UPDATE @UserIds SET Searched = 1 WHERE UserId = @UserId
    UPDATE @Emails SET Searched = 1 WHERE Email = @Email

    -- clear variables
    SET @UserId = NULL;
    SET @Email = NULL;

    -- reset variables to the next unsearched value
    SELECT TOP 1  @UserId = UserId
    FROM @UserIds
    WHERE Searched = 0;

    SELECT TOP 1 @Email = Email
    FROM @Emails
    WHERE Searched = 0;

  END

  -- display related records
  SELECT * FROM @Ids