SQL & PL/SQL

Hi

I'm developping an application using Pro C* in a C program. I have a problem, I would like to use a CASE WHEN statement inside a SELECT statement. I try the following code with compilation error:
EXEC SQL PREPARE pr_avor_annul FROM
INSERT INTO avor (NO_AVOR,MT_TOTL_AVOR,CD_DEVS,AC_STAT_AVOR,NO_FACT,MT_FRAS,
MT_ARTC,AC_AVOR,NO_COMP,MT_FRAS_CONT_REMB,DT_AVOR)
SELECT :vavor, mt_totl_fact + cout_livr + nvl(mt_fras_cont_remb,0), cd_devs,:vemis ,no_fact,cout_livr , mt_totl_fact, :vacavor, :vcomp, mt_fras_cont_remb,DECODE( (SELECT 0 FROM fact WHERE TO_CHAR(dt_fact)<TO_CHAR(sysdate)
AND no_fact= :vfact),dt_fact, sysdate, dt_fact)
FROM fact
WHERE no_fact = :vfact;

This code seems to be wrong but I don't know where...

Somebody can help me??

Thans a lot