Question

CREATE TABLE Items
(
  ItemId INT NOT NULL,
  CategoryId INT NOT NULL,
  ItemValue INT NOT NULL
)

A table contains items. Every item belongs to a category and stores some numeric value.

As a result of a query, I want each row to represent a category. There should be three columns:

• category id;
• id of an item with the highest ItemValue per category;
• id of an item with the lowest ItemValue per category.

Performance of the query is important. I use MS SQL Server.

Answer 1

For SQL Server 2005+, you can do this:

;WITH CTE AS
(
    SELECT  *, 
            ROW_NUMBER() OVER(PARTITION BY CategoryId ORDER BY ItemValue DESC) MaxRN,
            ROW_NUMBER() OVER(PARTITION BY CategoryId ORDER BY ItemValue) 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;

Here is a fiddle where you can test this solution.