visto che i mesi sono sempre dodici, puoi provare una query così
codice:
SELECT IMPORTO AS GEN, 0 AS FEB, 0 AS MAR, 0 AS APR, 0 AS MAG, 0 AS GIU, 0 AS LUG, 0 AS AGO, 0 AS SEP, 0 AS OTT, 0 AS NOV, 0 AS DIC FROM TABELLA WHERE MESE=1
UNION
SELECT 0 AS GEN, IMPORTO AS FEB, 0 AS MAR, 0 AS APR, 0 AS MAG, 0 AS GIU, 0 AS LUG, 0 AS AGO, 0 AS SEP, 0 AS OTT, 0 AS NOV, 0 AS DIC FROM TABELLA WHERE MESE=2
UNION
... ECCETERA
UNION
SELECT 0 AS GEN, 0 AS FEB, 0 AS MAR, 0 AS APR, 0 AS MAG, 0 AS GIU, 0 AS LUG, 0 AS AGO, 0 AS SEP, 0 AS OTT, 0 AS NOV, IMPORTO AS DIC FROM TABELLA WHERE MESE=12