CASE expressions using values from different rows

Question

I have below table

WITH TMP AS (
SELECT 'XX' AS AGG, 50 AS VALUE, 0 AS MONTH
UNION ALL
SELECT 'XX' AS AGG, 150 AS VALUE, 1 AS MONTH
UNION ALL
SELECT 'XX' AS AGG, 300 AS VALUE, 2 AS MONTH
UNION ALL
SELECT 'YY' AS AGG, 25 AS VALUE, 0 AS MONTH
UNION ALL
SELECT 'YY' AS AGG, 50 AS VALUE, 1 AS MONTH
UNION ALL
SELECT 'YY' AS AGG, 75 AS VALUE, 2 AS MONTH
UNION ALL
SELECT 'ZZ' AS AGG, 500 AS VALUE, 0 AS MONTH
UNION ALL
SELECT 'ZZ' AS AGG, 600 AS VALUE, 1 AS MONTH
UNION ALL
SELECT 'ZZ' AS AGG, 700 AS VALUE, 2 AS MONTH
)
-- CASE WHEN VALUE OF 'XX' > 0 THEN VALUE OF 'ZZ' ELSE 0 END
SELECT 
*
FROM TMP
WHERE 1 = 1

I need to apply following business logic to it always considering the month.

CASE WHEN VALUE OF 'XX' > 0 THEN VALUE OF 'ZZ' ELSE 0 END

This logic should be mapped into a new table. The result should look like the table below.
I would like to avoid joins to the same table. Is this somehow possible using window functions or something similar? The idea is to build several case when statement as the one above within the same sql statement.

IE1	MONTH
500	00
600	01
700	02

Answer 1

Pivot (conditionally aggregate) your data, and then return the value you need based on your expression:

WITH TMP AS
    (SELECT 'XX' AS AGG,
            50 AS VALUE,
            0 AS MONTH
     UNION ALL
     SELECT 'XX' AS AGG,
            150 AS VALUE,
            1 AS MONTH
     UNION ALL
     SELECT 'XX' AS AGG,
            300 AS VALUE,
            2 AS MONTH
     UNION ALL
     SELECT 'YY' AS AGG,
            25 AS VALUE,
            0 AS MONTH
     UNION ALL
     SELECT 'YY' AS AGG,
            50 AS VALUE,
            1 AS MONTH
     UNION ALL
     SELECT 'YY' AS AGG,
            75 AS VALUE,
            2 AS MONTH
     UNION ALL
     SELECT 'ZZ' AS AGG,
            500 AS VALUE,
            0 AS MONTH
     UNION ALL
     SELECT 'ZZ' AS AGG,
            600 AS VALUE,
            1 AS MONTH
     UNION ALL
     SELECT 'ZZ' AS AGG,
            700 AS VALUE,
            2 AS MONTH),
--Solution
/*WITH */Pvt AS(
    SELECT Month,
           MAX(CASE Agg WHEN 'XX' THEN VALUE END) AS XX,
           --MAX(CASE Agg WHEN 'YY' THEN VALUE END) AS YY,--Not needed hence commented out
           MAX(CASE Agg WHEN 'ZZ' THEN VALUE END) AS ZZ
    FROM TMP
    GROUP BY MONTH)
SELECT CASE WHEN XX > 0 THEN ZZ ELSE 0 END AS IE1,
       MONTH
FROM Pvt;