MS access replace null or zero values with nearest non-zero row

Question

I have a sample dataset like this

 Group  Age Value
    A   1   
    A   2   0
    A   3   20
    B   1   0
    B   2   0
    B   3   25
    B   4   0
    B   5   0
    B   6   50

For each distinct value in Group , replace null or zero Value . Ideal output will be

 Group  Age Value
    A   1   20
    A   2   20
    A   3   20
    B   1   25
    B   2   25
    B   3   25
    B   4   25
    B   5   50
    B   6   50

UPDATE

This seems to do the trick

UPDATE test
SET [Value] = nz(DMin(
    "[Value]", 
    "test", 
    "[Group] = '" & [Group] & "' AND 
     [Age] < " & [Age] & " AND 
     [Value] <> 0 AND [Value] IS NOT NULL"
),
 DMax(
    "[Value]", 
    "test", 
    "[Group] = '" & [Group] & "' AND 
     [Age] > " & [Age] & " AND 
     [Value] <> 0 AND [Value] IS NOT NULL"
))
WHERE [Value] = 0 OR [Value] IS NULL;

Answer 1

You can use this (in MsAccess SQL a column can be derived from a subquery returning or one value - or no rows):

select 
     tactl.[Group], 
     tactl.Age, 
     (  select trepl.[Value]
        from test trepl
        where tactl.[Group]=trepl.[Group]
              and nz(trepl.[Value],0)<>0
              and abs(tactl.Age-trepl.Age)=
                      ( select MIN(abs(ta.Age-tb.Age)) as MinDistanceFound 
                        from    test ta
                                inner join
                                test tb
                                on (ta.[Group]=tb.[Group]  and nz(tb.[Value],0) <> 0)
                       where (ta.[Group]=tactl.[Group]  and ta.[Age]=tactl.[Age])
                    )
      )  as [Value]
from test tactl
        

GROUP and VALUE can be reserved words in most SQL implementations (they are in Access); try to use other words for them (I put them in brackets to overcome issues)

Answer 2

Consider domain aggregation such as DMin in order to have an updateable query:

UPDATE test
SET [Value] = DMin(
    "[Value]", 
    "test", 
    "[Group] = '" & [Group] & "' AND 
     [Age] > " & [Age] & " AND 
     [Value] <> 0 AND [Value] IS NOT NULL"
)
WHERE [Value] = 0 OR [Value] IS NULL;