gistfile1.txt

The idea here is that there are two different rasters - one large rasterA, and one small rasterB used as a mask.  We need to get the values of rasterA which are within the bounds of rasterB. 

Additional problem: rasterB has NODATA value of 0, and ONLY the part of the rasterB that is NOT NODATA should be used as a mask.

The example below is wrong, for several reasons. Can you fix it?

select row_to_json(t) from (
    SELECT A.rid, B.rid, pvc
    FROM rasterA A 
    JOIN rasterB B 
    ON ST_Intersects(A.rast, B.rast), 
       ST_ValueCount(A.rast,1) AS pvc
) as t;