Question

n00b questioner here. I’m trying to do a query that checks if the most recent activity is within the last 24 hours. I technically can get the result I want, but the inequality in my case statement has to be in the opposite direction as would make sense to me. Here’s my query:

SELECT sqs.registration_id,
       MAX(sqs.completed_at)           AS 'most recent activity',
       DATE_SUB(NOW(), INTERVAL 1 DAY) AS 'one day ago',
       'recent activity?' = CASE
                              WHEN MAX(sqs.completed_at) < 
                                         DATE_SUB(NOW(), INTERVAL 1 DAY) 
                              THEN 1
                              ELSE 0
                            END
FROM   student_quiz_states sqs
WHERE  sqs.score = 100
GROUP  BY sqs.registration_id  

Here’s an example result:

XXXXX   2011-08-02 16:23:53 2011-12-05 00:06:05 0

This user did not have activity in the last 24 hours, so the last value returns 0, as I want it to.

However, that doesn’t make any sense to me. Shouldn’t the case statement return a 1, since the first datetime is much earlier than one day ago? It would make sense to me if my desired results were returned when my when_clause contained a > instead of a <.

Any explanations would be appreciated.

Answer 1

The problem is that your query contains 'recent activity?' = CASE ... END where it should have CASE ... END AS 'recent activity?'. The former is an equality-test rather than an expression with an alias. The reason for the seemingly “inverted” behavior is that, since the CASE expression is numeric (it evaluates to 0 or 1), MySQL performs a numeric equality-test, by converting 'recent activity?' to 0.0 (detailed rules here), such that 'recent activity?' = CASE ... END is true when the CASE expression gives 0 and false when it gives 1. Since MySQL represents true as 1 and false as 0, the end result is the opposite of what you were expecting.

(Note: An earlier version of this answer, while making the same basic point about equality-tests vs. aliases, and about false and true being 0 and 1, was vague/confused in other respects, since I didn’t recognize that the DBMS was MySQL, and was not aware that some DBMSes allow single-quotes to be used when quoting aliases. So if some of the comments above and below seem a bit strange, it’s because they’re referring to that version. The current state of the answer is thanks in large part to those comments.)