sql statement to delete records older than XXX as long as there are more than YY rows

delete
from
  MyTable
where
  item_id in
  (
    select
      item_id
    from
      MyTable
    group by
      item_id
    having
      count(item_id) > 15
  )
  and
    Date < @tDate

DELETE
    FROM    CommentTable
    WHERE   CommentId NOT IN (
        	SELECT	TOP 15 CommentId
        	FROM	CommentTable
        	WHERE	ItemId=@ItemId
        	AND	CommentDate < @Date
            ORDER BY CommentDate DESC
    )
    AND ItemId=@ItemId
    AND CommentDate < @Date

DELETE
    [MyTable]
WHERE
    [item_id] = 100 and
    (SELECT COUNT(*) FROM [MyTable] WHERE [item_id] = 100) > 15