选择when和value after then而不是string

是的,有可能:

select id, 
  case when exists (select data_od from bp_stan_produkt 
                  where id_produkt = bp_produkt.id and data_do is null 
                   and id_stan_produkt = 313) 
   then  (select to_char(data_od, 'YYYY-MM-DD')
                   from bp_stan_produkt 
                   where id_produkt = bp.id 
                     and data_do is null
                     and id_stan_produkt = 313)
  else 'NIE'
  end as "313"
from bp_produkt 
where id in(21048528);

编辑:

SELECT bp.id, COALESCE(t.data_od_t, 'NIE') AS "313"
FROM bp_produkt bp
LEFT JOIN LATERAL (select to_char(data_od, 'YYYY-MM-DD') AS data_od_t 
                   from bp_stan_produkt 
                   where id_produkt = bp.id 
                     and data_do is null
                     and id_stan_produkt = 313) t 
 ON TRUE
WHERE bp.id in(21048528)