Create view from two tables with subtract operation

Question

I'm almost new with SQL syntax and I need help to create a view on MySQL.

I have a table with a PK column called ID , a column called total_seats and another one is title . In the second table, I have multiple rows, with a firstname column and a FK that corresponds to the PK ( total_seats ) present in the first table.

I need to create a view where I can calculate the available_seats ( total_seats minus occurrence in the second table) for each element present in the first table.

Actually I'm calculating the "occupied" seats but the join give me the result only for already taken event, so the result is that I don't see the available_seats for the empty event.

SELECT b.ID_event, a.*,
COUNT(*) AS occupied FROM second_table b
LEFT JOIN first_table a ON b.ID_event = a.ID
GROUP BY ID_event

Answer 1

You could subtract the count

select a.ID_event, a.total_seats, count(*) as occupied,  a.total_seats - count(*) difference
from first_table a 
left join second_table ba ON b.ID_event = a.ID
group by  a.ID_event, a.total_seats