Q:

Converting DB2 Function to SQL Server Function

I am in the process of converting a bunch of function from DB2 to SQL server and this one has got me stuck, I've converted the function from this:
create function prf_t_excinter_nzw(AC_ID_i int, TFH_i double) specific prf_t_excinter_nzw   
RETURNS TABLE(ft_id int,rg_id int,thr double,value double        )  
R1: BEGIN ATOMIC
RETURN  select l.ft_id, l.rg_id, r.thr,     case when l.from is null      
then to_value else ( thr - from ) / ( to - from ) * ( to_value - from_value ) + from_value end as value        
from (     select ft_id, rg_id, to, from, to_value, from_value     from    (     select       row_number() over ( 
partition by l.ft_id, l.rg_id, l.thr order by l.ft_id, l.rg_id, l.thr, r.thr desc) as #,
l.ft_id, l.rg_id, l.thr as to, r.thr as from, l.value as to_value, r.value as from_value
from     (select ft_id, rg_id, thr, value from table ( prf_t_ftexc_nzw( AC_ID_i, TFH_i)) as d     
order by  ft_id, rg_id, thr asc     ) as l     
left outer join     (      select ft_id, rg_id, thr, value       from table ( prf_t_ftexc_nzw( AC_ID_i, TFH_i)) as d order by ft_id, rg_id, thr asc) as r     
on l.ft_id = r.ft_id and l.rg_id = r.rg_id and      l.thr > r.thr    ) as l_r    where # = 1  ) as l  join  (   select prm_id, thr from prf_t_prm_1d_thr as r 
where prm_id in ( select id from prf_t_prm where name like '%RG_NZW')    ) as r  on l.rg_id = r.prm_id and( l.from < r.thr or l.from is null) and  ( r.thr <= l.to )  
order by l.ft_id, l.rg_id, r.thr  ;  END

to this:
create function prf_t_excinter_nzw(@AC_ID_i int,@TFH_i float)
RETURNS TABLE
RETURN  select l.ft_id, l.rg_id, r.thr,case when l.from_value is null then to_value else ( thr-from_value )/(to_value-from_value)*(to_value-from_value)+from_value end as value
from (
select ft_id,rg_id,to_value,from_value,to_value,from_value 
from (
    select row_number() over ( partition by l.ft_id, l.rg_id, l.thr order by l.ft_id,l.rg_id, l.thr, r.thr desc) as num, l.ft_id, l.rg_id, l.to_value, r.from_value
    from (
        select ft_id, rg_id, thr, ac_value as to_value 
        from prf_t_ftexc_nzw(@AC_ID_i,@TFH_i) as d) as l 
        left outer join (
            select ft_id, rg_id, thr, ac_value as from_value
            from prf_t_ftexc_nzw(@AC_ID_i,@TFH_i) as d) as r 
            on l.ft_id = r.ft_id 
            and l.rg_id=r.rg_id 
            and l.thr > r.thr) as l_r 
            where num = 1) as l 
            join (
                select prm_id, thr from prf_t_prm_1d_thr as r 
                where prm_id in (select id from prf_t_prm where name like '%RG_NZW')) as r  
                on l.rg_id=r.prm_id and (l.from_value<r.thr or l.from_value is null) 
                and (r.thr<=l.to_value)

At this point i am getting the error Msg 8156, Level 16, State 1, Procedure prf_t_excinter_nzw, Line 18
The column 'to_value' was specified multiple times for 'l'
any tips/suggestions to fix?

A:

The 5th line is definitely generating an error:
select ft_id,rg_id,to_value,from_value,to_value,from_value 

You have to_value and from_value listed twice in the SELECT list. This would be fine if not a subquery, but since it's a subquery you need unique names for each field, and in this case they're redundant so the duplicates just need to be removed.
The error message points you to the l object which is the alias you've given the subquery that starts with the offending SELECT line mentioned above.
These issues become easier to ferret out if you don't re-use aliases for different objects in a query, since there are a few things labeled l here.

