SQL Server Query - groupwise multiplication

Question

If we have a table like this:

Grp       Value

Grp1          2
Grp1          5
Grp1          3
Grp2          3
Grp2         -5
Grp2         -2
Grp3                   


        

Answer 1

At does annoy me that there is no Multiply / Product aggregate function in SQL server. Unfortunately I didn't find the answer @gbn gave above until I'd already solved the problem a different way. I'm posting this alternative solution just in case it helps anyone, or if it turns out to be more efficient.

My solution basically involves using a recursive common table expression to multiply all the values together.

DECLARE @t TABLE (PID INT, ID INT, multiplier DECIMAL(14,5))

INSERT @t
        (PID, ID, multiplier)
VALUES  (1, 1, 1.5 )
      , (2, 1, 1.2)
      , (3, 2, 1.7)
      , (4, 3, 1)
      , (5, 4, 0.8)
      , (6, 4, 0.5)
      , (7, 4, 2)
      , (8, 4, 0.5)

SELECT *, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY PID )
FROM @t;


WITH 
trn AS
(
SELECT PID, ID, multiplier, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY PID ) AS rn
FROM @t
),
tmu AS 
(
    SELECT DISTINCT 
        ID, 
        trn.multiplier AS multiplier,
        1 AS Iteration
    FROM trn WHERE rn = 1
    UNION ALL 
    SELECT 
        trn.ID, CAST(tmu.multiplier * trn.multiplier AS DECIMAL(14,5)) AS multiplier
        , tmu.Iteration + 1 AS Iteration
    FROM
        tmu --AS tmu1
        JOIN trn ON tmu.ID = trn.ID AND tmu.Iteration + 1 = trn.rn 
),
mxi AS
(
    SELECT ID, COUNT(ID) AS Iter
    FROM trn
    GROUP BY ID
)

SELECT tmu.* 
FROM tmu 
     JOIN mxi ON mxi.ID = tmu.ID AND mxi.Iter = tmu.Iteration
ORDER BY ID

Starting table (+ the partitioned row number) selects to:

PID ID  Multiplier  rn
1   1   1.50000 1
2   1   1.20000 2
3   2   1.70000 1
4   3   1.00000 1
5   4   0.80000 1
6   4   0.50000 2
7   4   2.00000 3
8   4   0.50000 4

List of ID / Multipliers all multiplied together selects to:

ID  multiplier  Iteration
1   1.80000 2
2   1.70000 1
3   1.00000 1
4   0.40000 4