使用 SQLite 将组中的行相乘 [英] Multiply rows in group with SQLite

问题描述

我有一个查询，它返回某个标记具有特定分类的概率.

I have a query that returns the probability that a token has a certain classification.

token       class       probPaired
----------  ----------  ----------
potato      A           0.5
potato      B           0.5
potato      C           1.0
potato      D           0.5
time        A           0.5
time        B           1.0
time        C           0.5

我需要通过将它们相乘来聚合每个 class 的概率.

I need to aggregate the probabilities of each class by multiplying them together.

-- Imaginary MUL operator
select class, MUL(probPaired) from myTable group by class;

class       probability
----------  ----------
A           0.25
B           0.5
C           0.5
D           0.5

如何在 SQLite 中执行此操作?SQLite 没有 LOG/EXP 或变量之类的功能 - 解决方案 在其他问题中提到.

How can I do this in SQLite? SQLite doesn't have features like LOG/EXP or variables - solutions mentioned in other questions.

推荐答案

您可以计算行数，然后使用递归 cte 进行乘法.然后获取包含乘法最终结果的每个类的最大 rnum(计算的 row_number)值.

You can calculate row numbers and then use a recursive cte for multiplication. Then get the max rnum (calculated row_number) value for each class which contains the final result of multiplication.

--Calculating row numbers
with rownums as (select t1.*,
                 (select count(*) from t t2 where t2.token<=t1.token and t1.class=t2.class) as rnum 
                 from t t1)
--Getting the max rnum for each class
,max_rownums as (select class,max(rnum) as max_rnum from rownums group by class)
--Recursive cte starts here
,cte(class,rnum,probPaired,running_mul) as
    (select class,rnum,probPaired,probPaired as running_mul from rownums where rnum=1
     union all
     select t.class,t.rnum,t.probPaired,c.running_mul*t.probPaired 
     from cte c
     join rownums t on t.class=c.class and t.rnum=c.rnum+1)
--Final value selection
select c.class,c.running_mul 
from cte c
join max_rownums m on m.max_rnum=c.rnum and m.class=c.class

SQL Fiddle

这篇关于使用 SQLite 将组中的行相乘的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！