MySQL LEFT JOIN issue: Retrieve workshops names in column A and a row id in column B - mysql

Question

Apologies for the rubbish question title, but it's a bit tricky to summarise my requirement into a single line. I usually don't have an issue with MySQL JOINs but this one is throwing me.
I'm building a training feedback system and for one feature would like to display a list of all available workshops in the database, which workshops a given delegate has been assigned to and whether any feedback has been submitted by that delegate for those assigned workshops.
I could do this in a couple of queries, but I'm trying to do something a bit more elegant with a single query.
The pertinent details of my database structure:
WORKSHOPS table
id: INT
name: TINYTEXT
DELEGATES table
id: INT
name: TINYTEXT
FEEDBACK table
delegate_id: INT
workshop_id: INT
feedback: TEXT
DELEGATES_X_WORKSHOPS table
delegate_id: INT
workshop_id: INT
delegate_id and workshop_id in the tables are Foreign Keys to the DELEGATES and WORKSHOPS tables.
As any given delegate can be assigned to multiple workshops, I'm using the DELEGATES_X_WORKSHOPS table as a cross-referencing table so I can quickly search for who is assigned to any given workshop or which workshops any given delegate is assigned to.
However, I've tried LEFT JOINing a couple of different ways and I can't get a full list of workshops on the left and matches (if they exist) on the right for a given delegate_id.
Example data
Delegate Ross has delegate_id = 1
Registered workshops are
C++
PHP
ASP.NET
HTML5
JavaScript
Ross is assigned to PHP, HTML5 and JavaScript
Question 1 is this: how do I return the following for delegate_id=1:
[workshop] | [assigned]
C++ | null
PHP | TRUE
ASP.NET | null
HTML5 | TRUE
JavaScript | TRUE
(it doesn't matter right now what goes into column B, I just want a null if a particular delegate_id hasn't been assigned to a workshop).
I've used this:
SELECT
workshops.name,
delegates_x_workshops.delegate_id
FROM
workshops
LEFT JOIN
delegates_x_workshops
ON
workshops.id=delegates_x_workshops.workshop_id
WHERE
delegates_x_workshops.delegate_id=1
However I'm only returning the 3 rows where delegate_id=1, not 5 rows for all workshops.
Question 2 is a bit more involved:
Taking question 1 as a base, how would I work column C to display if feedback has been left for a workshop that Ross has been assigned to?
[workshop] | [assigned] | [givenfeedback]
C++ | null | null
PHP | TRUE | TRUE
ASP.NET | null | null
HTML5 | TRUE | null
JavaScript | TRUE | TRUE
Thanks in advance to anybody who makes it this far and has a clue what I'm blithering about. As I said, I could rattle through this with a few different queries, but I'm trying to keep things elegant.
No doubt half of this will need clarification, so ask any questions.
Thanks

Answer 1

For question 1, you need to move the where condition into the on clause. It is turning the left outer join into an inner join because non-matching rows have NULL values:
SELECT w.name, dxw.delegate_id
FROM workshops w LEFT JOIN
delegates_x_workshops dxw
ON w.id = dxw.workshop_id and
dxw.delegate_id = 1;
For the second question, I think this is what you want:
SELECT w.name,
(case when max(w.name = 'Ross') > 0 then 'True' end) as Assigned,
(case when count(f.workshop_id) > 0 then 'True' end) as Feedback
FROM workshops w LEFT JOIN
delegates_x_workshops dxw
ON w.id = dxw.workshop_id and
dxw.delegate_id = 1 LEFT JOIN
delegates d
on d.id = dxw.delegate_id LEFT JOIN
feedback f
on f.workshop_id = w.id
GROUP BY w.name;

Answer 2

For reference, here's my final query:
SELECT DISTINCT
workshops.id AS wid,
workshops.name AS workshop,
(delegates_x_workshops.delegate_id IS NOT NULL) AS assigned,
(initial_feedback.delegate_id IS NOT NULL
OR
ongoing_feedback.delegate_id IS NOT NULL) AS hasfeedback
FROM
workshops
LEFT JOIN
delegates_x_workshops
ON
workshops.id = delegates_x_workshops.workshop_id
AND
delegates_x_workshops.delegate_id = 1
LEFT JOIN
initial_feedback
ON
workshops.id = initial_feedback.workshop_id
AND
initial_feedback.delegate_id = 1
LEFT JOIN
ongoing_feedback
ON
workshops.id = ongoing_feedback.workshop_id
AND
ongoing_feedback.delegate_id = 1
ORDER BY
workshop ASC
For every workshop in the WORKSHOPS table, I'll get the id and name of the workshop, 1 or 0 if a given delegate_id is assigned and 1 or 0 if feedback of either type (I have 2 kinds) has been left for that workshop.
Scary to think that all I was missing was an AND condition on my LEFT JOIN.
Thanks again Gordon!