續集：如何使用組和計數

Question

簡單地說，我怎樣才能使用Sequel來做這個查詢？

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

您的問題是，左連接找到每個相冊ID的曲目ID。解決方案：

right join

子查詢款項，假設續集支持：left join (select album_id, count(album_id) as count from tracks group by album_id) t on

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

Answer 2

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