CREATE TABLE [#sales]
     (
      [dt]          [INT], 
      [StoreID]     [INT], 
      [GoodID]      [INT], 
      [Price]       [FLOAT], 
     );
...
WHILE .... (цикл по датам понедельников за год - около 50)
  UPDATE A
     SET Price = a.Price - b.Price
     FROM [#sales] A
    INNER JOIN [#sales] B ON a.StoreID = b.StoreID AND a.GoodID = b.GoodID AND b.dt = @dt
   WHERE a.dt > b.dt

             UPDATE [snp]
               SET 
                   [DevReg] =   IIF([snp].[Price] = 0, 0, ROUND(([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price], 2)), 
                   [DevPromo] = IIF([snp].[Price] = 0, 0, ROUND(([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price], 2)), 
                   [isPromo] =  IIF([snp].[Price] <> 0
                                    AND ([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price] < -0.11
                                    AND ([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price] < 0.23, 
								1, 0), 
                   [PriceReg] = IIF([snp].[Price] <> 0
                                    AND ([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price] < -0.11
                                    AND ([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price] < 0.23, 
								 [snpPrev].[PriceReg], [snp].[Price]), 
                   [PricePromo] = IIF([snp].[Price] <> 0
                                     AND ([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price] < -0.11
                                     AND ([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price] < 0.23, 
								  [snp].[Price], [snpPrev].[PricePromo])
             FROM [#salesPromo] AS [snp]
             -- Поиск значений предыдущей записи
                  INNER JOIN [#salesPromo] AS [snpPrev] ON 1 = 1
                                  AND [snp].[dt] > [snpPrev].[dt]
                                  AND [snpPrev].[dt] = @dt
                                  AND [snp].[StoreID] = [snpPrev].[StoreID]
                                  AND [snp].[GoodID] = [snpPrev].[GoodID]
								  AND snp.ChildGoodID = snpPrev.ChildGoodID
             WHERE [snp].[dt] BETWEEN @dtnext AND @curperiodend

    UPDATE [snp]
               SET 
                   [DevReg] =   IIF([snp].[Price] = 0, 0, ROUND(([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price], 2)), 
                   [DevPromo] = IIF([snp].[Price] = 0, 0, ROUND(([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price], 2)), 
                   [isPromo] =  IIF([snp].[Price] <> 0
                                    AND ([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price] < -0.11
                                    AND ([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price] < 0.23, 
								1, 0), 
                   [PriceReg] = IIF([snp].[Price] <> 0
                                    AND ([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price] < -0.11
                                    AND ([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price] < 0.23, 
								 [snpPrev].[PriceReg], [snp].[Price]), 
                   [PricePromo] = IIF([snp].[Price] <> 0
                                     AND ([snp].[Price] - [snpPrev].[PriceReg]) / [snp].[Price] < -0.11
                                     AND ([snp].[Price] - [snpPrev].[PricePromo]) / [snp].[Price] < 0.23, 
								  [snp].[Price], [snpPrev].[PricePromo])
             FROM [#salesPromo] AS [snp]

            OUTER APPLY (SELECT TOP 1 [dt], [StoreID], [GoodID], [ChildGoodID], 
				                  ISNULL([Price], 0) [Price],ISNULL([DevReg], 0) [DevReg],
						  ISNULL([DevPromo], 0) [DevPromo],
                                                  ISNULL([isPromo], 0) [isPromo],
						  ISNULL([PriceReg], 0) [PriceReg],
						  ISNULL([PricePromo], 0) [PricePromo] 				  
				  FROM [#salesPromo] AS b 
                                  WHERE 1 = 1
                                            AND b.[dt] >= @startweek AND b.[dt] < [snp].[dt]
                                            AND [snp].[StoreID] = b.[StoreID]
                                            AND [snp].[GoodID] = b.[GoodID]
				           AND snp.ChildGoodID = b.ChildGoodID
				  ORDER BY b.Dt DESC
                                ) [snpPrev]
             WHERE [snp].[dt] = @dt and [snpPrev].dt IS NOT NULL

DECLARE @T TABLE (dt int, val int)
INSERT @T VALUES (20200101, 100), (20200108, NULL), (20200115, NULL), (20200122, 120)
SELECT *, LAG(Val) OVER(ORDER BY dt) as lastvalue
FROM @T