SQL Join Issue 一对多关系

Question

I have two tables product and reservations, I want retrieve all the products that aren't not included into a date range, I'm using a left join to reach the related reservations to a specific product, the matter is that I must filter by date range, and return all the products that aren't included in that range, there are several products, and in most of the cases the products aren't related to any reservation, I'm kinda lost with this:

select p.idProduct, p.name, p.description, p.latitude, p.longitude, p.address,
             p.qualification, p.favourite,p.idCategory,p.idCity, p.rules, p.health, p.politics,
             r.idReservation
             from product p 
            LEFT JOIN city c ON c.idCity = p.idCity 
            LEFT JOIN reservation r ON r.idProduct = p.idProduct 
            where p.idCity = 1 
            AND ( 
            (r.endDate < '2021-11-20' AND r.startDate< '2021-11-28') OR 
            (r.endDate > '2021-11-28' AND r.startDate > '2021-11-20') 
            );

In this case for example there are not reservations on those dates, but there are multiple products, that doesn't have a relationship with any reservation.

One product could be related to zero or many reservations.

I will appreciate any hint about it.

Answer 1

IF what do you mean (it's not so clear what do you need or your logic) is that reservation is not mandatory, then your conditions doesn't have to be mandatory. Move them to LEFT JOIN

select p.idProduct, p.name, p.description, p.latitude, p.longitude, p.address,
             p.qualification, p.favourite,p.idCategory,p.idCity, p.rules, p.health, p.politics,
             r.idReservation
             from product p 
            LEFT JOIN city c ON c.idCity = p.idCity 
            LEFT JOIN reservation r ON r.idProduct = p.idProduct AND ( 
            (r.endDate < '2021-11-20' AND r.startDate< '2021-11-28') OR 
            (r.endDate > '2021-11-28' AND r.startDate > '2021-11-20') 
            )
            where p.idCity = 1 
            ;