If record exist in A then return record else if record exist in B return record ELSE NULL

WITH Captains_CTE (TeamName, PlayerName)
AS (
    SELECT TeamName, PlayerName FROM TeamPlayer WHERE IsCaptain = 1
),
OldestPlayer_CTE (TeamName, PlayerName)
AS (
    SELECT TeamName, PlayerName FROM TeamPlayer AS tp
    INNER JOIN (
        SELECT TeamName, MAX(PlayerAge) AS MaxAge
        FROM TeamPlayer
        GROUP BY TeamName
    ) AS old ON old.TeamName = tp.TeamName AND old.MaxAge = tp.PlayerAge

)
SELECT  CASE
            WHEN Captains_CTE.TeamName IS NULL THEN OldestPlayer_CTE.TeamName
            ELSE Captains_CTE.TeamName
        END AS TeamName,
        CASE
            WHEN Captains_CTE.PlayerName IS NULL THEN OldestPlayer_CTE.PlayerName
            ELSE Captains_CTE.PlayerName
        END AS PlayerName
FROM Captains_CTE
FULL OUTER JOIN OldestPlayer_CTE ON Captains_CTE.TeamName = OldestPlayer_CTE.TeamName

select a.*
from a
union all
select b.*
from b
where not exists (select 1 from a where a.? = b.?);  -- "?" is for the column that specifies whether the record exists in A