a query question

hi all,

i was trying to migrate some data into a new structure i created. so i
execute this exact query:

insert into rec_t_ordenpago("ent_codigo", "sec_codigo", "ani_codigo",
"orp_numero",
"orp_tipopersona", "orp_persona", "cic_codigo",
"esp_codigo", "cur_codigo", "cur_paralelo",
"rub_codigo", "orp_valor", "orp_fechaemision",
"orp_fechavencimiento", "ban_codigo",
"cta_numero",
"est_codigo")
select cua.ent_codigo, cua.sec_codigo, cua.ani_codigo,
(select (nvl(max(orp_numero), 0) + 1) from rec_t_ordenpago),
'AL', alcu.alu_codigo, cua.cic_codigo, cua.esp_codigo,
cua.cur_codigo,
cua.cur_paralelo, cua.rub_codigo, cua.cpa_valor, current_date,
cua.cpa_fechavencimiento, null, null, 'PE'
from rec_m_cuadropagos cua, aca_t_alumnocurso alcu
where cua.ent_codigo = 1
and alcu.ent_codigo = cua.ent_codigo
and alcu.sec_codigo = cua.sec_codigo
and alcu.ani_codigo = cua.ani_codigo
and alcu.cic_codigo = cua.cic_codigo
and alcu.esp_codigo = cua.esp_codigo
and alcu.cur_codigo = cua.cur_codigo
and alcu.cur_paralelo = cua.cur_paralelo

but i ended up with duplicate key error then i replace the (select
(nvl(max(orp_numero), 0) + 1) from rec_t_ordenpago) part with
nextval('seq1') and that work.
So my question is, is this a postgreSQL limitation or is the way it have to
happen? why?

thanx in advance,

Jaime Casanova

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