对于您的代码值，这就可以了。

SELECT COUNT(DSTINCT record_company) number_of_labels, cod_singer
              FROM recording
             GROUP BY cod_singer

然后你可以加入歌手的行列。。。

SELECT s.singer_name, n.number_of_labels
   FROM singer s
   LEFT JOIN (
            SELECT COUNT(DSTINCT record_company) number_of_labels, cod_singer
              FROM recording
             GROUP BY cod_singer
        ) n ON s.cod_singer = n.cod_singer

专业提示：学习如何使用groupby和各种聚合函数（如count（*）和count（distinct column））所花的每一个小时都是一个很好的时间。sql在这方面提供了巨大的能力。

你可以做：

select name
from (
  select
    s.cod_singer, 
    max(c.name_singer) as name,
    count(distinct c.record_company) as labels
  from singer s
  join recording r on r.cod_singer = s.cod_singer
  join record_company c on c.cod_son = r.cod_song
  group by s.cod_singer
) x
order by labels desc
limit 1