MySQL - 两个表和两个连接

Question

I am stuck with one query where I want to do several things: - I have a table with some events, each event has a limited amount of people that can apply to it - In other table I keep record of who applied to which event - On the website, to the user, I want to display only the events that he can apply to (to be more precise, the events he hasn't applied to; once he applies, the event should no longer be visible to the user) AND/OR that number of applied people is less than max_no_people (when an event is full, no need to show it to the user

I got help from here in another thread, but only for part of the problem. Tables look like this:

other_events (eventID, max_no_people, active)
event_applied (eventID, userID)

Here is the query:

SELECT count(event_applied.eventid) AS no_applied, 
       e.eventID, e.max_no_people, e.active 
FROM other_events e 
     INNER JOIN 
     event_applied ON event_applied.eventID = e.eventID 
     LEFT JOIN 
     event_applied ea ON ea.eventID 
                      AND event_applied.userID = :userID 
WHERE ea.eventID IS NULL 
      AND e.active = 1
GROUP BY event_applied.eventID 
HAVING (no_applied < max_no_people)

This query works fine - BUT - for every other user than the currently selected one. Let's say you have userID = 42 (which is not in events_applied table paired with eventID = 123):

other_events

eventID | max_no_people | active
--------|---------------|-------
   21   |       5       |   1

and in events_applied

eventID | userID
--------|-------
123     |  10
123     |  11
123     |  12
123     |  13
123     |  14

The Query will return empty row (because event is maxed out and you cannot apply anymore), but if you are userID = 12 (which is in the events_applied table and has eventID pair), the result will look like this:

no_applied | e.eventID | e.max_no_people | e.active
-----------|-----------|-----------------|---------
 **4**     |     123   |         5       |    1

and hence, the event will be visible and you will be able to apply again and again (I know I have to add some additional controls, but I'll tackle that later).

So, the problem is that this query somehow discards the row of a current user and doesn't add it into final count.

Anybody? :)

Answer 1

The problem is that in your current query you are only explicitly excluding the entries where the User has applied, and therefore the count is reduced by one. You instead need to exclude all the events where they applied.

Since you're using a LEFT JOIN you'll need to use a subquery to select all those events and then only join on the events (and not join on the user).

SELECT count(event_applied.eventid) AS no_applied, e.eventID, e.max_no_people, e.active 
FROM other_events e 
INNER JOIN event_applied ON event_applied.eventID = e.eventID 
LEFT JOIN (SELECT * FROM event_applied WHERE event_applied.userID = :userID ) ea 
ON ea.eventID = e.eventID 
WHERE ea.eventID IS NULL AND e.active = 1
GROUP BY e.eventID, e.max_no_people, e.active
HAVING (no_applied < max_no_people)

Answer 2

The main issue with your query is that you are not correlating the left join ed query on a specific eventID .

Consider the following code:

SELECT 
    COUNT(*) AS no_applied, 
    e.eventID, 
    e.max_no_people, 
    e.active 
FROM 
    other_events e 
    INNER JOIN event_applied ea1 ON ea1.eventID = e.eventID 
    LEFT JOIN event_applied ea2 ON ea1.eventID = ea2.eventID AND ea2.userID = :userID
WHERE ea2.eventID IS NULL AND e.active = 1
GROUP BY e.eventID, e.max_no_people, e.active 
HAVING (no_applied < e.max_no_people)

If you run this query in this DB Fiddle with your sample data:

• with userID = 42 , it returns no record: this is because there is no event available (there is only one event, which is already full)

• with userID = 12 , it returns no record: this is because this user already applied to the only available event

I would suggest creating a more representative dataset, for example by creating another event that is not full, so you can thouroughly validate the query.

Answer 3

Thank you both Graeme Tate and GMB , both approaches work perfectly: I learned something new today :)