通过Postgresql计算正确的汇率

Question

Here'e the table result 's data:

question_id is_correct count
64          TRUE        11
66          FALSE       20
77          FALSE       15
77          TRUE        9

I need to calculate the correct rate, which equals (TRUE_COUNT)/(TRUE_COUNT+FALSE_COUNT) . The problem for now is that some questions miss TRUE or FALSE data, I want to fill it with zero in order to do the self join operation and do the calculation. Is there any way to fulfill the requirement or fill with zero? Great thanks!

Expected result:

question_id correct_rate
64          1
66          0
77          0.625

Answer 1

No need for a self-join, you can do conditional aggregation:

select
   question_id,
   -- to return a percentage
   100.0 * 
   max(case when is_correct = TRUE then count else 0 end) /
   sum(count)

   -- otherwise needs to add a cast if "count" is an INT
   -- max(case when is_correct = TRUE then count else 0 end) /
   -- cast(sum(count) as decimal(5,4))

from tab
group by question_id

Answer 2

Conditional aggregation is correct but there are a couple of nuances:

SELECT
  question_id
  ,SUM(CASE WHEN is_correct THEN count ELSE 0 END)
    /(CASE WHEN SUM(count) < 1 THEN 1 ELSE SUM(count) END)::NUMERIC
FROM
  result
GROUP BY
  question_id

Make sure you don't end up in a divide by 0 (in case of 0/0), and ensure you cast to a decimal or numeric to get your desired rate.

Also note your desired result of .625 is not correct for 77 that would be .375 9 true / 24 total

Answer 3

You should not need a JOIN at all. Check out below on how to conditionally aggregate your data:

SELECT
    question_id,
    COALESCE(SUM(count) FILTER(WHERE is_correct),0) / SUM(count)::NUMERIC AS rate
FROM
    result
GROUP BY
    question_id;

-- Older version of PostgreSQL without FILTER:

SELECT
    question_id,
    COALESCE(SUM(CASE WHEN is_correct THEN count END),0) / SUM(count)::NUMERIC AS rate
FROM
    result
GROUP BY
    question_id;