union und sum problem bei mysql

Ich habe ein Problem mit union (über 3 Tabellen), die Ausgabe sollte Spaltensummen ergeben, also 1 Zeile mit den Summen einiger Spalten. Die Ausgabe besteht aber aus 3 Zeilen den Summen der 3 Tabellen:

$query=mysql_query("select
YEAR(dat) as j, SUM(betrag_brutto) as suma, SUM(ust_betrag) as sumv, 0 as sumu
from a
union
select YEAR(dat) as j, 0 as suma, 0 as sumv, SUM(ust_betrag) as sumu
from b
union
select YEAR(dat) as j,0 as suma, 0 as sumv, SUM(sumust) as sumu,
from c
group by YEAR(dat)") or die(mysql_error());

while($DS = mysql_fetch_array( $query ))
{ ...

Ergebnis:
suma: 2.529,88, sumv: 0,00, sumu: 356,09
suma: 0,00, sumv: 15,00, sumu: 0,00
suma: 0,00, sumv: 30,22, sumu: 0,00

angestrebtes Ergebnis:
suma: 2.529,88, sumv: 45,22, sumu: 356,09

ich glaube die Lösung gefunden zu haben: getestet hab ich noch nicht, ich stelle - wenn ich es zs bringe - danach die richtige Version rein
$query=mysql_query("
select j, sum(suma), sum(sumv), sum(sumu)
from
(select YEAR(dat) as j, SUM(betrag_brutto) as suma, SUM(ust_betrag) as sumv, 0 as sumu
from a group by YEAR(dat)
union
select YEAR(dat) as j, 0 as suma, 0 as sumv, SUM(ust_betrag) as sumu
from b group by YEAR(dat)
union
select YEAR(dat) as j,0 as suma, 0 as sumv, SUM(sumust) as sumu,
from c group by YEAR(dat)
) as d
group by j
") or die(mysql_error());

Leider ist das auch nicht die Lösung, nach wie vor 3 Zeilen, aber auch ohne die gewünschten Summen ... weiss jemand den Fehler, den es hier hat, die Konstruktion erscheint mir nämlich richtig ...?

Lösung einfach dargestellt:
select j, sum (a) as sa, sum(b) as sb
from
(select j, xa as sa, 0 as sb
from tab1
union
(select j, ya as sa, yb as sb
from tab2)
as tab3
group by j

Lösung:
$query=mysql_query("
select j, sum(suma) as suma, sum(sumv) as sumv, sum(sumu) as sumu
from
(select YEAR(dat) as j, betrag_brutto as suma, ust_betrag as sumv, 0 as sumu
from a
union
select YEAR(dat) as j, 0 as suma, 0 as sumv, ust_betrag as sumu
from b
union
select YEAR(dat) as j,0 as suma, 0 as sumv, sumust as sumu,
from c
) as d
group by j
") or die(mysql_error());