codice:
select id,substring_index(group_concat(campo order by campo),',',1) as minore from (
select id,campo1 as campo from tabella where campo1 > 0
union all
select id,campo2 from tabella where campo2 > 0
union all
select id,campo3 from tabella where campo3 > 0
union all
select id,campo4 from tabella where campo4 > 0
union all
select id,campo5 from tabella where campo5 > 0
) as t
group by id