SQL:Hur man sorterar värden i kategorier i ssms

Du verkar vilja:

SELECT ItemRelation, DocumentNum, DocumentYear, IsPromo
FROM [dbo].[promo_data]
WHERE CustomerType = 'XY' AND IsPromo = 1
GROUP BY ItemRelation, DocumentNum, DocumentYear, IsPromo
HAVING COUNT(*) = 1;

EDIT:

För att hämta alla kolumner, använd fönsterfunktioner:

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;