在另一个表中存在的记录的计算比例

Question

您能否帮我解决unknownComputation中的SQLite声明如下？

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

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

并非来自T1的所有URL都在T​​2和我想找到这些比率，并将它们按t1.attr

Answer 1

随着信息你提供的，也许这应该这样做：

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，此认为，一个T1可以有0个或多个T2

Answer 2

假设T1有一个名为id独特的ID，请尝试：

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