Select 表中的最新时间戳值，该表具有列 id 的多个条目，每个唯一列 id 和来自另一个表的数据

Question

I need to draw data from two tables for a school site. Users flag (tFlag) topics they have studied and are ready to answer questions on. These choices are recorded in the userTopics table. There are 55 topics (topicID), and there are many questions available (qID) for each topic on the site.

The UserTopics table also lists a rolling rating of their performance, the topicScore, for each topic. There will only be one unique row for any user per topicID.

UserTopics

| ID | userID | topicID | tFlag | topicScore |
| 25 | 1      | 1       | 1     | 25         |
| 29 | 1      | 2       | 1     | 70         |
| 42 | 1      | 3       | 0     | 5          |
| 41 | 1      | 5       | 0     | 5          |
| 35 | 1      | 6       | 1     | 43         |
| 31 | 1      | 7       | 1     | 62         |
| 44 | 1      | 8       | 0     | 0          |
| 32 | 1      | 9       | 0     | 5          |
| 35 | 1      | 12      | 1     | 30         |

The Results table logs the results of each answered question, and timestamps the answer time, so the table can have many records for each question answered by the user, and looks like this (other cols removed)

Results

| resultID | userID | topicID | qID | correct | answerTime          |
| 9        | 1      | 12      | 15  | 1       | 2020-05-28 11:29:18 |
| 10       | 1      | 12      | 26  | 1       | 2020-05-28 11:30:18 |
| 11       | 1      | 1       | 132 | 0       | 2020-06-02 17:03:42 |
| 13       | 1      | 2       | 50  | 0       | 2020-06-02 17:02:53 |
| 14       | 1      | 7       | 10  | 1       | 2020-06-02 17:05:15 |
| 76       | 1      | 6       | 9   | NULL    | 0000-00-00 00:00:00 |
| 75       | 1      | 6       | 9   | NULL    | 0000-00-00 00:00:00 |
| 65       | 1      | 9       | 12  | NULL    | 0000-00-00 00:00:00 |
| 66       | 1      | 9       | 12  | 1       | 2020-06-04 07:34:02 |

In order to have the system automatically assign a question for the student, we need values for topicScore and answerTime for a specific user for every topicID that they are currently learning. One row must be returned for each topicID in the UserTopics table where tFlag = 1 for the user. The most recent answerTime from the Results table for each returned topicID from userTopics should then be fetched. If, however, the only answerTime listed for a particular topicID is 0000-00-00 00:00:00, I need that returned. (the answerTime col uses ON UPDATE CURRENT_TIMESTAMP, so if they have loaded the question but not answered it, the value here is zero.)

A query result for userID = 1 would hopefully be:

Query result

| topicID | topicScore | answerTime          |
| 1       | 25         | 2020-06-02 17:03:42 |
| 2       | 70         | 2020-06-02 17:02:53 |
| 6       | 43         | 0000-00-00 00:00:00 |
| 7       | 62         | 2020-06-02 17:05:15 |
| 12      | 30         | 2020-06-04 07:34:02 |

I have tried the following query, but it's not getting what I want, and also don't actually understand it as my grasp of mySQL is a bit basic atm. (query written by another)

SELECT
    r.userID,
    r.topicID,
    r.answerTime,
   (SELECT t.topicScore FROM UserTopics t WHERE t.userID = r.userID AND t.topicID = r.topicID) AS topicScore
    FROM Results r
    LEFT JOIN Results r2 ON r2.topicID = r.topicID AND r.answerTime < r2.answerTime
    WHERE r2.answerTime IS NULL AND r.userID = 1 
    ORDER BY `r`.`topicID` ASC

I can see it needs to have Where t.tFlag =1 somewhere, but when I put it in the bracketed where clause, it doesn't work either so I assume the whole query needs a re-write. Glad for any help.

Answer 1

Here's the query I have now tried and tested following O Jones suggestions below....needed a couple of changes but the core was just what the doctor ordered, works like a charm, thanks. Needed only one userID per query, and only results where topicScores are not 0. Also as there is only one row per topicID in the UserTopics table, didn't need the MAX statement either.

SELECT ut.topicId, 
       ut.topicScore,
       MAX(r.answerTime) answerTime
  FROM UserTopics ut
  LEFT JOIN Results r   ON ut.userId = r.userId
                       AND ut.topicId = r.topicId
WHERE ut.tFlag =1 AND ut.userId = 1 AND ut.topicScore >0
GROUP BY ut.topicId 
ORDER BY ut.topicId

Answer 2

First of all, using zero datestamps makes for a brittle design. Later versions of MySQL disallow zero datestamps. You'll need to give this command to make things work, that is to remove the NO_ZERO_DATE mode.

SET sql_mode='ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';

You'd be better off using NULL rather than a zero datestamp. That being said...

This is a job for GROUP BY , where you pull out the maximum score and answerTime for each combination of userId and topicId. ( https://www.db-fiddle.com/f/swC1jG2mMHRsAkCouJNfXq/5 )

SELECT ut.userId, ut.topicId, 
       MAX(ut.topicScore) topicScore, 
       MAX(r.answerTime) answerTime
  FROM UserTopics ut
  LEFT JOIN Results r   ON ut.userId = r.userId
                       AND ut.topicId = r.topicId
WHERE ut.tFlag =1
GROUP BY ut.userId, ut.topicId 
ORDER BY ut.userId, ut.topicId