SELECT ...
GROUP BY b.CodContract

La ce anume se refera acel GROUP BY de la coada? Nu ai nici o functie agregat in select, deci nu ai de ce sa ai GROUP BY

In general ce vrei tu se face mai usor si mai rapid printr-un self join, nu prin subquery. Asa pe ghicite as zice ca ceva de genul asta:

select b.CodContract
from LBF B
join (select NodId, CodContract, sum(principalpaltit+principalrestant-amendamentprincipal) as sum
 from lbf
 where tipcredit=0
 and datareferinta='12/31/2008'
 group by codcontract
) s on b.codcontract=s.codcontract and b.nodid=s.nodid
where datareferinta='12/31/2008'
and tipcredit=0
and b.valoarecontract != s.sum;

Join-ul poate fi outer join, depinde de ce anume vrei.

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