Exécution de la multiplication dans T-SQL

La solution suivante suppose qu'il y a toujours 3 lignes par trimestre (seul le dernier trimestre peut être partiel), un seul SELECT, pas de récursivité :-)

WITH sumQuart AS
 (
   SELECT *,
      CASE
        WHEN ROW_NUMBER() -- for the 1st month in a quarter
             OVER (PARTITION BY CCP, Years, Quart
                   ORDER BY months) = 1
                  -- return the sum of all GTS of this quarter
        THEN SUM(GTS) OVER (PARTITION BY CCP, Years, Quart)
        ELSE NULL -- other months
      END AS sumGTS
   FROM gts
 )
,cte AS
 (
   SELECT 
      sq.*,
      COALESCE(b.Baseline, -- 1st quarter
               -- product of all previous quarters
               CASE
                 WHEN MIN(ABS(sumGTS)) -- any zeros?
                      OVER (PARTITION BY sq.CCP ORDER BY sq.Years, sq.Quart, sq.Months 
                            ROWS BETWEEN UNBOUNDED PRECEDING AND 3 PRECEDING) = 0 
                   THEN 0  
                 ELSE -- product
                      EXP(SUM(LOG(NULLIF(ABS(COALESCE(b.Baseline,1) * sumGTS),0)))
                          OVER (PARTITION BY sq.CCP ORDER BY sq.Years, sq.Quart, sq.Months 
                                ROWS BETWEEN UNBOUNDED PRECEDING AND 3 PRECEDING)) -- product
                      -- odd number of negative values -> negative result
                    * CASE WHEN COUNT(CASE WHEN sumGTS < 0 THEN 1 END) 
                                OVER (PARTITION BY sq.CCP ORDER BY sq.Years, sq.Quart, sq.Months 
                                      ROWS BETWEEN UNBOUNDED PRECEDING AND 3 PRECEDING) % 2 = 0 THEN 1 ELSE -1 END
               END) AS newBaseline
   FROM sumQuart AS sq
   LEFT JOIN BASELINE AS b
          ON B.CCP = sq.CCP
          AND b.Quart = sq.Quart
          AND b.Years = sq.Years
 )
SELECT 
   CCP, months, Quart, Years, GTS,
   round(newBaseline * GTS,2),
   round(newBaseline,2)
FROM cte

Voir Violon

EDIT :Ajout d'une logique pour gérer les valeurs <=0 Fiddle