MySQL - Join tables, retrieve only Max ID

问题:

I've seen solutions for something similar on other posts, but I've been having an issue applying it to my specific problem.

Here is my initial join:

SELECT service_note_task, comment_id, comment FROM service_note_task LEFT JOIN service_note_task_comments ON service_note_task.service_note_task_id = service_note_task_comments.service_note_task_id;  

Which results in:

+-----------------------------+------------+--------------+
| service_note_task           | comment_id | comment      |
+-----------------------------+------------+--------------+
| This is service note task 3 |         25 | Comment      |
| This is service note task 3 |         26 | Comment Blah |
| This is service note task 3 |         36 | aaa          |
| This is service note task 2 |         13 | Awesome comm |
| This is service note task 1 |         12 | Cool Comm    |
+-----------------------------+------------+--------------+

But for each service_note_task, I really only need one row representing the comment with the highest comment_id, like this:

+-----------------------------+------------+--------------+
| service_note_task           | comment_id | comment      |
+-----------------------------+------------+--------------+
| This is service note task 3 |         36 | aaa          |
| This is service note task 2 |         13 | Awesome comm |
| This is service note task 1 |         12 | Cool Comm    |
+-----------------------------+------------+--------------+

I figure I could use MAX in a sub-select statement to narrow down the results as I want them. How can I incorporate that into my statement to get these results?

回答1:

For reference, this is known as "groupwise-maximum"

http://dev.mysql.com/doc/refman/5.0/en/example-maximum-column-group-row.html

回答2:

since you haven't mention the RDBMS you are using, this query below mostly works on many RDBMS (not all)

SELECT  a.*, b.*   -- select only the columns you want. 
FROM    service_note_task a
        INNER JOIN service_note_task_comments b
            ON a.service_note_task_id = b.service_note_task_id
        INNER JOIN 
        (
            SELECT  service_note_task_id, MAX(commentID) max_ID
            FROM    service_note_task_comments
            GROUP   BY service_note_task_id
        ) c ON  b.service_note_task_id = c.service_note_task_id AND
                b.commentID = c.max_ID

if your RDBMS supports Analytical Functions, you can use this below,

SELECT  a.service_note_task, b.comment_id, b.comment 
FROM    service_note_task a
        INNER JOIN 
        (
            SELECT  service_note_task_id, comment_id, comment,
                    ROW_NUMBER() OVER (PARTITION BY service_note_task_id
                                        ORDER BY comment_id DESC) rn
            FROM    service_note_task_comments
            GROUP   BY
        ) c ON  a.service_note_task_id = b.service_note_task_id AND
                b.rn = 1

回答3:

try:

SELECT service_note_task, comment_id, comment 
FROM service_note_task SNT1 
LEFT JOIN service_note_task_comments ON service_note_task.service_note_task_id = service_note_task_comments.service_note_task_id
WHERE comment_id = (SELECT MAX(comment_id) FROM  service_note_task SNT2 WHERE SNT1.service_note_task = SNT2.service_note_task);

回答4:

SELECT service_note_task, comment_id, comment
FROM service_note_task s LEFT JOIN service_note_task_comments sc
  ON s.service_note_task_id = sc.service_note_task_id;
WHERE EXISTS (
              SELECT 1
              FROM service_note_task_comments s2
              WHERE s.service_note_task_id = s2.service_note_task_id
              HAVING MAX(s2.comment_id) = sc.comment_id
              )