query based on row number for psql8.3 - Mailing list pgsql-sql

Hello,

For psql8.3, is there a simple way to query based on row number please?

For example,

select  case                    when row_num=1 THEN    t1.tot_hr                    ELSE
 t1.tot_hr - 
 
t1.pre_hr          END
FROM  t1 ;