MySQL联接-检查具有某些列值的不存在的行

Question

I have a restaurants table and a bookings table. Each booking stores a date, number of persons, and one of 3 time slots ( event_time ). An example row:

id | restaurant_id |         date         | number_of_persons | event_time
7        2           2019-09-12 00:00:00            15               2

I want to return all restaurants that has free seats bookable for any event_time on a certain date.

Restaurants with no bookings are returned using left join. I can get bookings with the join and do SUM(number_of_persons) and check if max capacity has been met for each event_time that has bookings. The problem is that any event_time having 0 bookings is not included in the result (since it is non existant), so it doesn't count.

The current query excludes restaurants which are fully booked for the event_times which has bookings. So if event_time 1 and 2 are fully booked, and event_time 3 is free and bookable, that restaurant is still excluded (wrongly).

Here is the current query:

select restaurants.id, `restaurants`.title,
    number_of_seats_max, 
    num_persons_booked, 
    number_of_seats_max - num_persons_booked AS free_seats_left,
    event_time
from `restaurants` 
left join ( select `restaurant_id`, `b`.`event_time`, 
     SUM(b.number_of_persons) as num_persons_booked 
    from `bookings` as `b` 
    where `event_date` = "2019-9-12" 
    group by `b`.`event_time`, `restaurant_id`
     order by num_persons_booked
 ) as `bookings_summaries` 
on `restaurants`.`id` = `bookings_summaries`.`restaurant_id`  
having number_of_seats_max - num_persons_booked > 0   // <-- not fully booked
or num_persons_booked IS NULL                        //<-- no bookings

I have a fiddle here.

The logic I think is needed is: Return restaurants which have bookings and:

• does not have bookings for all the three event_times

or

• have bookings for all three event_times and there are still available seats.

But I don't know how to implement it. I appreciate any help!

Answer 1

You need to assign event_time 1,2,3 for every place before left join with booking_summerise then only it gives results for every event.

select restaurants.id, `restaurants`.title,
    number_of_seats_max, 
    restaurants.event_time,
    IFNULL(num_persons_booked,0) AS num_persons_booked, 
    number_of_seats_max - IFNULL(num_persons_booked,0) AS free_seats_left
from ((select *,("1") AS event_time from restaurants) UNION ALL (select *,("2") AS event_time from restaurants) UNION ALL (select *,("3") AS event_time from restaurants)) AS restaurants 
left join ( select `restaurant_id`, `b`.`event_time`, SUM(b.number_of_persons) as num_persons_booked 
    from `bookings` as `b` 
    where `event_date` = "2019-9-12" 
    group by `b`.`event_time`, `restaurant_id`
     order by num_persons_booked
 ) as `bookings_summaries` 
on `restaurants`.`id` = `bookings_summaries`.`restaurant_id` AND 
`restaurants`.`event_time` = `bookings_summaries`.`event_time` 
having number_of_seats_max - num_persons_booked > 0
or num_persons_booked IS NULL
ORDER BY `restaurants`.`id`

DEMO