That is easy:

decode( line_amt, 0, 'what it should be when line_amt=0',
'what it should be when line_amt<>0' );


I believe:


decode( acc_type, 'Y',
decode( rev_type, 'BASE',
decode( line_amt, 0, NULL,
decode( l_type, 'LEASE', line_quantity,
'RENT', line_quantity,
NULL
)
),
NULL ),
NULL )


will do it -- returns NULL is the predicate is not satisfied. It says:

if ( acc_type = 'y' )
then
if ( rev_type = 'base' )
then
if ( line_amt = 0 )
then
return null;
else
if ( l_type = 'lease' ) or ( l_type = 'rent' )
then
return line_quantity;
else
return null;
end if;
end if;
else
return null;
end if;
else
return null;
end if;