Question

I have a database with two tables, events and tickets. The events table contains rows of events, and tickets each ticket sold.

For example,

events
------
id name  capacity
1  Test  10
2  Test2 20
3  Test3 15

tickets
-------
id event_id customer_id
1  1        (value here doesn't matter)
2  1
3  3
4  1
5  3

So I can see that 3 tickets to event #1 have been sold, 0 tickets to event #2, and 2 tickets to event #3 have been sold.

I’m trying to create a query that selects all the events, plus a column available which is the events capacity less the count of tickets sold.

I have this query:

SELECT
events.id,
events.name,
events.capacity,
events.capacity - COUNT(tickets.id) AS available
FROM
events
INNER JOIN tickets ON events.id = tickets.event_id

However, this doesn’t return any events where tickets havn’t yet been sold. I thought it might be the type of join, but I tried various ones with no success.

I’m not sure where I’m going wrong.

Answer 1

You’re almost there, just change the inner join to a left join:

SELECT  events.id,
,       events.name
,       events.capacity
,       events.capacity - COUNT(tickets.id) AS available
FROM    events
LEFT JOIN 
        tickets 
ON      events.id = tickets.event_id
group by
        events.id,
,       events.name
,       events.capacity

Even though MySQL does not require it, it’s good practice to list all non-aggregated columns explicitly in the group by clause.