with tmp( vremja, priznak ) as 
(values (current date,  1 ), (current date +  1  day,  2 ), (current date +  2  day,  3 )) 
select 
	   case
	   	   when priznak =  1  then vremja
		   when priznak =  2  then vremja +  1  month
		   else null
	   end as vremja 
from tmp;

with tmp( vremja, priznak ) as 
(values (current date,  1 ), (current date +  1  day,  2 ), (current date +  2  day,  3 )) 
select 
	   case
	   	   when priznak =  1  then vremja
		   when priznak =  2  then vremja +  1  month
		   else SIGNAL SQLSTATE '75100' SET MESSAGE_TEXT = 'error'
	   end as vremja 
from tmp;

[IBM][CLI Driver][DB2/NT] SQL0104N  An unexpected token "SQLSTATE '75100' SET MESSAGE_TEXT = '...