Shedal

WITH CTE AS
(
  SELECT
    *, 
    ROW_NUMBER() OVER(PARTITION BY CategoryId ORDER BY ItemValue DESC) MaxRN,
    ROW_NUMBER() OVER(PARTITION BY CategoryId ORDER BY ItemValue ASC) MinRN
  FROM Items
)
SELECT
  CategoryId, 
  MIN(CASE WHEN MaxRN = 1 THEN ItemId END) IdMaxValue,
  MIN(CASE WHEN MinRN = 1 THEN ItemId END) IdMinValue
FROM CTE
GROUP BY CategoryId