在 SQL 中找到最大的分数差异

Question

I am trying to write a query to find the largest point differential for students before and after taking a course. I need the user_id of the person with the largest score differential for every course (there are around 500).

The columns that are provided in the table include: id, user_id, course_id, #_questions_correct, #_questions, and quiz_type. The options pre, and post exist in the quiz_type column.

I cannot seem to find a way to do this efficiently, so I would love to hear suggestions on making this easier!

So far all I have come up with is:

SELECT user_id, course_id (number_correct / number_of_questions)*100 AS test_score, quiz_type
FROM results
WHERE quiz_type = 'pre'
ORDER BY test_score;

I don't know if this is a good start or what, let me know!

Answer 1

This should work. Note you might need to make syntax changes - I don't know postgresql so this is formatted for SQL Server.

SELECT user_id, course_id, postscore-prescore AS differential
FROM
    --Get all the prescores
    (
        SELECT user_id, course_id, (1.0)*num_questions_correct/num_questions AS prescore
        FROM @input
        WHERE quiz_type = 1 --quiz_type = pre
    ) AS a
    INNER JOIN
    --Get all the postscores
    (
        SELECT user_id AS tmp, course_id AS tmp2, (1.0)*num_questions_correct/num_questions AS postscore
        FROM @input
        WHERE quiz_type = 2 -- quiz_type = post
    ) AS b
    --Put the prescores and postscores in the same table as separate columns
    ON a.user_id=b.tmp AND a.course_id=b.tmp2
ORDER BY differential DESC