在另一個表中存在的記錄的計算比例

Question

您能否幫我解決unknownComputation中的SQLite聲明如下？

select t1.attr, unknownComputation as ratio 
from t1 
group by t1.attr 
order by ratio 

unknownComputation應該(count(t1.attr) for which there exists t2.url = t1.link)/count(t1.attr)

並非來自T1的所有URL都在T​​2和我想找到這些比率，並將它們按t1.attr

Answer 1

隨着信息你提供的，也許這應該這樣做：

select t1.attr, count(t2.url)/count(t1.attr) as ratio 
from t1 
left join t2 on t2.url = t1.link 
group by t1.attr 
order by ratio 

但是，此認爲，一個T1可以有0個或多個T2

Answer 2

假設T1有一個名爲id獨特的ID，請嘗試：

select t1.attr, count(t2.url)/count(distinct t1.id) as ratio 
from t1 
left join t2 on t2.url = t1.link 
group by t1.attr 
order by 2