IF OBJECT_ID('tempdb.dbo.#Data') IS NOT NULL
    DROP TABLE #Data

SELECT *
     , ROW_NUMBER() OVER (PARTITION BY [item], code ORDER BY [data]) - ROW_NUMBER() OVER (PARTITION BY [item], code, action ORDER BY [data]) AS grp
INTO #Data
FROM test_median

--CREATE CLUSTERED INDEX ix ([item], code, data) -- хз

;WITH CTE AS (
    SELECT *
    FROM #Data c
    OUTER APPLY (
        SELECT TOP(1) t.grp AS prvgrp
        FROM #Data t
        WHERE t.action = 0
            AND t.data <= c.data
            AND t.[item] = c.[item]
            AND t.code = c.code
        ORDER BY t.data DESC
    ) prev
    OUTER APPLY (
        SELECT TOP(1) t.grp AS nxtgrp
        FROM #Data
        WHERE t.action = 0
            AND t.[item] = c.[item]
            AND t.code = c.code
            AND t.data >= c.data
        ORDER BY t.data
    ) nxt
)

SELECT c.item
     , c.code
     , c.data
     , c.sales
     , c.action
     , CASE
           WHEN action = 1 AND
               med < sales THEN med
           ELSE sales
       END AS output
FROM CTE1 c
OUTER APPLY (
    SELECT MAX(med) AS med
    FROM (
        SELECT PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY sales) OVER (PARTITION BY [item], code, grp) AS med
        FROM #Data
        WHERE grp IN (nxtgrp, prvgrp)
            AND [item] = c.[item]
            AND code = c.code
    ) r
) c2
ORDER BY data