GROUP BY and GROUP_CONCAT - how to return correct values?

Question

I have the following query:

SELECT ev.q1, ev.comments, es.session_number, es.title, CONCAT( np.first_name,  ' ', np.last_name ) AS speaker
FROM  `expo_session_eval` ev
LEFT JOIN expo_session es ON es.session_id = ev.session_id
LEFT JOIN expo_session_speaker ess ON ess.session_id = ev.session_id
LEFT JOIN expo_speaker sp ON sp.speaker_id = ess.speaker_id
LEFT JOIN new_people np ON np.id = sp.people_id
GROUP BY CONCAT( np.first_name,  ' ', np.last_name ) 
ORDER BY es.session_number

This returns data that looks like this:

session_id  q1  comments    session_number  title               speaker
===================================================================================
169         3   Good!       103            Digital Practices      Steve Bullock
169         3   Good!       103            Digital Practices      Sheila Bacon
170         1               104            LBS = Location Based   Patrick Moorhead

This is correct in that there are two records in expo_session_eval for session_id 169, but it's incorrect in that the q1 and comments values are not identical. There are two records in expo_session_speaker that correspond to session_id 169 - ie there are two speakers for this one session.

Ideally, I'd like my results to look like this:

session_id  q1  comments    session_number  title                  speaker
===============================================================================================
169         3   Good!       103            Digital Practices        Steve Bullock, Sheila Bacon
169         5   Great!      103            Digital Practices        Steve Bullock, Sheila Bacon
170         1               104            LBS = Location Based     Patrick Moorhead

I've tried using GROUP_CONCAT, but apparently I'm not using it correctly, because when I use this query:

SELECT ev.session_id, ev.q1, ev.comments, es.session_number, es.title, GROUP_CONCAT( CONCAT( np.first_name,  ' ', np.last_name ) ) AS speaker
FROM  `expo_session_eval` ev
LEFT JOIN expo_session es ON es.session_id = ev.session_id
LEFT JOIN expo_session_speaker ess ON ess.session_id = ev.session_id
LEFT JOIN expo_speaker sp ON sp.speaker_id = ess.speaker_id
LEFT JOIN new_people np ON np.id = sp.people_id
ORDER BY es.session_number

I get this:

session_id  q1  comments    session_number  title                  speaker
===============================================================================================
169         3   Good!       103         Digital Practices   Steve Bullock,Sheila Bacon,Patrick Moorhead

What do I need to do to make the speaker column group_concat within the session_id?

EDITS FOR @NOAH

Main table is expo_session_eval - each record has a session_id stored. These IDs come from a table called expo_session. There's a cross-reference table called expo_session_speaker that contains only two columns - session_id and speaker_id. There's another table called speaker_id that contains speaker_id and people_id. people_id links the table to a table called new_people, which contains first_name and last_name.

In expo_session_speaker, there can be multiple records for a given session_id - each record corresponds to a single session and a single speaker. So to display ALL speakers for a given session, you end up with multiple records. That's where the group by/group_concat idea came in; I need to display ALL the speakers for a given session in ALL records for that session. Hence, session 169 record 1 speakers are Steve Bullock, Sheila Bacon, and session 169 record 2 speakers are the same, even though none of the other selected values will be the same for the two session 169 records.

(Clear as mud, right?)

Answer 1

You should group by session_id (instead of ordering)

SELECT ev.session_id, ev.q1, ev.comments, es.session_number, es.title, GROUP_CONCAT(  CONCAT( np.first_name,  ' ', np.last_name ) ) AS speaker
FROM  `expo_session_eval` ev
LEFT JOIN expo_session es ON es.session_id = ev.session_id
LEFT JOIN expo_session_speaker ess ON ess.session_id = ev.session_id
LEFT JOIN expo_speaker sp ON sp.speaker_id = ess.speaker_id
LEFT JOIN new_people np ON np.id = sp.people_id
GROUP BY ev.session_id

Answer 2

Something like this should do what you want -

SELECT ev.session_id, ev.q1, ev.comments, es.session_number, es.title, speaker
FROM  `expo_session_eval` ev
LEFT JOIN expo_session es ON es.session_id = ev.session_id
LEFT JOIN (
    SELECT ess.session_id, GROUP_CONCAT( CONCAT( np.first_name,  ' ', np.last_name ) ) AS speaker
    FROM new_people np
    LEFT JOIN expo_speaker sp ON  np.id = sp.people_id
    LEFT JOIN expo_session_speaker ess ON sp.speaker_id = ess.speaker_id
    GROUP BY ess.session_id
) np ON np.session_id = es.session_id   
ORDER BY es.session_number

You don't actually want to use a GROUP BY statement on the entire result set, just to create a concatenated string of the speakers so it needs to be done in a subquery.