select a.id_a
, a.description
, coalesce( group_concat(distinct concat(b.description, '[', c.quantity, ']') order by b.id_b separator ', ')
, '')
from a
left join c on a.id_a = c.idex_a
left join b on b.id_b = c.idex_b
group by a.id_a
, a.description