sql to find number of attendees (joined) of an event - mysql

Question

I'm trying to find number of attendees who have actually joined the sessions of the event. I've 5 tables.
event -> day -> session
attendee
session_attendee
Schema
+------------------------------+
| attendee |
+------------------------------+
| id username password |
| 1 user1 test |
| 2 user2 test |
| ------ |
| event |
| id name |
| 1 event 1 |
| 2 event 2 |
| ----- |
| day |
| id date event_id |
| 1 '2015-06-01' 1 |
| 2 '2015-06-02' 1 |
| 3 '2015-07-01' 2 |
| 4 '2015-07-02' 2 |
| ------ |
| session |
| id name day_id |
| 1 session a 1 |
| 2 session b 1 |
| 3 session c 2 |
| 4 session d 2 |
| ------ |
| session_attendee |
| id session_id attendee_id |
| 1 1 1 |
| 2 2 1 |
| 3 1 2 |
| 4 2 2 |
+------------------------------+
Expectation
+-----------------------------+
| id name attendees |
+-----------------------------+
| 1 'event 1' 2 |
| 2 'event 2' 0/null etc. |
+-----------------------------+
Subquery
SQL query I've tried using Joins but it's returning more than one record (i.e. I can't use it as a subquery)
SELECT count(*) FROM event
INNER JOIN day ON event.id = day.event_id
INNER JOIN session ON day.id = session.day_id
INNER JOIN session_attendee ON session.id = session_attendee.session_id
WHERE event.id = 1
GROUP BY attendee_id
Parent Query
with above subquery substituted
SELECT id, name,
(SELECT COUNT(*) FROM day WHERE day.event_id = event.id) AS days,
(SELECT COUNT(*) FROM event
INNER JOIN day ON event.id = day.event_id
INNER JOIN session ON day.id = session.day_id
INNER JOIN session_attendee ON session.id = session_attendee.session_id
GROUP BY session_attendee.attendee_id) AS attendees
FROM event;
SQL Fiddle
http://sqlfiddle.com/#!9/72ffc
More Work:
SELECT event.id, event.name,
(SELECT count(*) FROM event e INNER JOIN day AS d ON e.id = d.event_id WHERE e.id = event.id) AS total_days,
day.date, session.name, session_attendee.attendee_id
FROM event
LEFT JOIN day ON event.id = day.event_id
LEFT JOIN session ON day.id = session.day_id
LEFT JOIN session_attendee ON session.id = session_attendee.session_id
ORDER BY event.id;
-- COUNT(distinct attendee_id) AS attendees
As soon as I enter COUNT(distinct attendee_id), my result set gets reduced to one row, i.e. I don't get to see records for other events.

Answer 1

Your first query looks basically right with one exception: if you want information for an event, you should aggregate by the event id, not the attendee id:
SELECT e.id, e.name, count(*)
FROM event e INNER JOIN
day d
ON e.id = d.event_id INNER JOIN
session s
ON d.id = s.day_id INNER JOIN
session_attendee sa
ON s.id = sa.session_id
GROUP BY e.id;
Of course, if you want information for only one event, you can replace the GROUP BY with:
WHERE e.id = 1
EDIT:
You want a LEFT JOIN to keep all the events:
SELECT e.id, e.name, count(distinct sa.attendee_id)
FROM event e LEFT JOIN
day d
ON e.id = d.event_id LEFT JOIN
session s
ON d.id = s.day_id LEFT JOIN
session_attendee sa
ON s.id = sa.session_id
GROUP BY e.id;

Answer 2

Try this
SELECT event.id, name,
(SELECT COUNT(*) FROM day WHERE day.event_id = event.id) AS days,
b.number_of_attendee
FROM event
left join (
SELECT a.id, count(*) as number_of_attendee from
(
SELECT event.id, attendee_id FROM event
INNER JOIN day ON event.id = day.event_id
INNER JOIN session ON day.id = session.day_id
INNER JOIN session_attendee ON session.id = session_attendee.session_id
group by event.id, attendee_id
) as a
group by a.id) as b
on b.id = event.id
which has the result like
id name days number_of_attendee
1 Event 1 2 2
2 Event 2 2 (null)