ST_BandNoDataValue — Returns the value in a given band that represents no data. If no band num 1 is assumed.
double precision ST_BandNoDataValue(raster rast, integer bandnum=1);
Returns the value that represents no data for the band
SELECT ST_BandNoDataValue(rast,1) As bnval1,
ST_BandNoDataValue(rast,2) As bnval2, ST_BandNoDataValue(rast,3) As bnval3
FROM dummy_rast
WHERE rid = 2;
bnval1 | bnval2 | bnval3
--------+--------+--------
0 | 0 | 0