create function first_col(sc varchar( 128 ), nm varchar( 128 ))
returns varchar( 128 )
language sql
begin atomic
declare cn varchar( 128 );
select column_name 
into cn
from qsys2.syscolumns
where table_schema=sc and table_name=nm
order by column_name
fetch first  1  row only;
return cn;
end;

select table_schema, table_name, first_col(table_schema, table_name) column_name
from qsys2.systables 
where table_schema='QSYS2' and table_name='SYSTABLES';