SQL only select rows with max date within each user - sql

Question

SQL beginner here. I've got a simple test that users take, and each row is the answer to one of their questions. They're allowed to take the exam once per day, so some people take it a second time on another day, and thus will have many rows with different test dates. What I'm basically trying to do is get each user's most recent score.
Here is what my data looks like (table name is dumdum):
+----------+----------------+----------+------------------+
| USERNAME | CORRECT_ANSWER | RESPONSE | DATE_TAKEN |
+----------+----------------+----------+------------------+
| matt | 1 | 1 | 3/23/15 1:04:26 |
| matt | 2 | 2 | 3/23/15 1:04:28 |
| matt | 3 | 3 | 3/23/15 1:04:23 |
| david | 1 | 3 | 3/20/15 1:04:25 |
| david | 2 | 2 | 3/20/15 1:04:28 |
| david | 3 | 1 | 3/20/15 1:04:30 |
| david | 1 | 1 | 3/21/15 11:03:14 |
| david | 2 | 3 | 3/21/15 11:03:17 |
| david | 3 | 2 | 3/21/15 11:03:19 |
| chris | 1 | 2 | 3/17/15 12:45:52 |
| chris | 2 | 2 | 3/17/15 12:45:56 |
| chris | 3 | 3 | 3/17/15 12:45:59 |
| peter | 1 | 1 | 3/19/15 2:45:33 |
| peter | 2 | 3 | 3/19/15 2:45:35 |
| peter | 3 | 2 | 3/19/15 2:45:38 |
| peter | 1 | 1 | 3/20/15 12:32:04 |
| peter | 2 | 2 | 3/20/15 12:32:05 |
| peter | 3 | 3 | 3/20/15 12:32:05 |
+----------+----------------+----------+------------------+
and what I'm trying to get in the end...
+----------+------------------+-------+
| USERNAME | MOST_RECENT_TEST | SCORE |
+----------+------------------+-------+
| matt | 3/23/2015 | 100 |
| david | 3/21/2015 | 33 |
| chris | 3/17/2015 | 67 |
| peter | 3/20/2015 | 100 |
+----------+------------------+-------+
I ran into some trouble because I need to go by day, and not by day/time, so I had to do a weird maneuver where I went to character and back to date... This is what I have so far, but I can't figure out how to use only the scores from the most recent test (right now it's factoring in all scores from every test ever taken)...
SELECT username, to_date(substr(max(test_date),1,9),'dd-MON-yy') as most_recent_test, round((sum(case when response=correct_answer then 1 end)/3)*100,0) as score
FROM dumdum group by username
Any help would be appreciated! Thanks!

Answer 1

There are several solutions to this problem this one uses the WITH clause and the RANK function.
It also uses the TRUNC function rather than to_date(substr(
with mxDate as
(SELECT USERNAME,
TRUNC(DATE_TAKEN) as MOST_RECENT_TEST,
CASE WHEN CORRECT_ANSWER = RESPONSE THEN 1 else 0 END as SCORE,
RANK () OVER (PARTITION BY USERNAME
ORDER BY TRUNC(DATE_TAKEN) DESC) Rk
FROM dumdum)
SELECT
USERNAME,
MOST_RECENT_TEST,
SUM(SCORE)/3 * 100
FROM
mxDate
WHERE
rk = 1
GROUP BY
USERNAME,
MOST_RECENT_TEST
Demo