Você pode fazer isso sem um caso:
SELECT Team.teamID
FROM Team
WHERE (@teamid = 0 AND Team.teamID > 0)
OR (@teamid <> 0 AND Team.teamID = @teamid)
SELECT Team.teamID
FROM Team
WHERE (@teamid = 0 AND Team.teamID > 0)
OR (@teamid <> 0 AND Team.teamID = @teamid)
