MySQL - Join tables, retrieve only Max ID - sql

Question

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?

Answer 1

For reference, this is known as "groupwise-maximum"
http://dev.mysql.com/doc/refman/5.0/en/example-maximum-column-group-row.html

Answer 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

Answer 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);

Answer 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
)