续集：如何使用组和计数

Question

简单地说，如何使用Sequel进行此查询？

select a.id, count(t.id)
from albums a
right join tracks t on t.album_id = a.id
group by a.id

Answer 1

DB[:albums___a].
  right_join(:tracks___t, :album_id=>:id).
  select_group(:a__id).
  select_more{count(:t__id)}

Answer 2

您的问题是左连接会为每个相册ID找到一个曲目ID。解决方案：

right join

总和的子查询，假设续集支持：left join (select album_id, count(album_id) as count from tracks group by album_id) t on

竖起from albums a, tracks t where t.album_id=a.id而不是加入。