あなたが望んでいるようです:
SELECT ItemRelation, DocumentNum, DocumentYear, IsPromo
FROM [dbo].[promo_data]
WHERE CustomerType = 'XY' AND IsPromo = 1
GROUP BY ItemRelation, DocumentNum, DocumentYear, IsPromo
HAVING COUNT(*) = 1;
編集:
すべての列を取得するには、ウィンドウ関数を使用します:
SELECT *
FROM (SELECT pd.*,
COUNT(*) OVER (PARTITION BY ItemRelation, DocumentNum, DocumentYear, IsPromo) as cnt
FROM [dbo].[promo_data] pd
WHERE CustomerType = 'XY' AND IsPromo = 1
) pd
WHERE cnt = 1;