Understanding GROUP BY with a LIMIT - mysql

Question

I've got a simple SQL query to give me information about all of the users who have commented on an image like this...
SELECT user.id, user.userName, user.displayName, user.currentDefault, user.discipline
FROM user
INNER JOIN imageComment ON user.id = imageComment.commentAuthorId
WHERE imageComment.imageAlbumJunctionId = 37
GROUP BY user.id
LIMIT 2
I have the LIMIT 2 on there because I know there are only two comments for this image. And I have the GROUP BY user.id on there because I only want to show information about a user once, even if they comment multiple times.
So lets say that "Mike" commented on a photo twice. My question is, does this query...
Only search for 2 comments because of the LIMIT, and then perform the GROUP BY user.id
Perform the GROUP BY user.id and then search the full table for a second unique user
I am hoping that this query does #1 because if it does #2 that would cause it to search the entire table looking for a second user when "Mike" was actually the one that did both comments. And I did try an EXPLAIN but I don't really understand it because it gives the same output whether there is a GROUP BY or LIMIT. Thank you for reading.

Answer 1

The query finds the first two users who have commented an Image, so it's #2.
I'd suggest:
select ...
from user
where exists
(
select * from imageComment
where imageComment.commentAuthorId = user.id
and imageComment.imageAlbumJunctionId = 37
)
where exists is faster than a inner join because it can stop after the first. Good indices should be set.

Answer 2

LIMIT is applied after the GROUP BY user.id. So in this case, #2 is happening. But the WHERE clause will filter the table first, so it's not searching the whole table. Your query will give you correct results, but I think this should be better:
SELECT DISTINCT user.id, user.userName, user.displayName, user.currentDefault, user.discipline
FROM user
INNER JOIN imageComment ON user.id = imageComment.commentAuthorId
WHERE imageComment.imageAlbumJunctionId = 37
LIMIT 2

Answer 3

Sample Tables
User Table imageComment Table
id | userName id | commentAuthorId | imageAlbumJunctionId
--------------- -------------------------------------------
1 | Mike 1 | 1 | 37
2 | John 2 | 1 | 37
3 | Carla 3 | 2 | 37
4 | 3 | 37
Split explanation about what going on
SELECT user.id, user.userName
FROM user
INNER JOIN imageComment ON user.id = imageComment.commentAuthorId
WHERE imageComment.imageAlbumJunctionId = 37
this will return
1 | Mike
1 | Mike
2 | John
3 | Carla
after your GROUP BY user.id you will now get
1 | Mike
2 | John
3 | Carla
now we use the LIMIT 2 and get
1 | Mike
2 | John