SQL Join，对吗？ 剩下 ？ 内？

Question

working with mySql I would like to list all purchases that customers made on a specific cathegory of products.

So, I had 3 tables: customers (idCustomer, Name) , cathegories (idCategory, CategoryName) and orders (idOrder, idCustomer, idCathegory, Qty, Price)

But I want a listing with ALL of the customers. Not only the one who bought that specific idCategory

I thought something like:

select sum(Orders.Qty), Customers.Name 
from Orders
right join Customers on Orders.idCustomer = Customer.idCustomer
where Orders.idCategory = 'Notebooks'
group by Orders.idCategory

but this statement only lists the records for customers who exists in Orders table. And I want all of them ( the one who didnt buy, with qty =0 )

thanks in advance

Answer 1

Most people find left join easier to follow than right join . The logic for left join is to keep all rows in the first table, plus additional information from the remaining tables. So, if you want all customers, then that should be the first table.

You will then have a condition on the second table. Conditions on all but the first table should be in the on clause rather than a where . The reason is simple: when there is no match, then the value will be NULL and the where condition will fail.

So, try something like this:

select sum(o.Qty) as sumqty, c.Name 
from Customers c left join
     Orders o
     on o.idCustomer = c.idCustomer and
        o.idCategory = 'Notebooks'
group by c.Name;

Finally, the group by should have a relationship to the select clause.

Answer 2

Try this query

select sum(Orders.Qty), Customers.Name 
from Customers 
right join Orders on Customer.idCustomer = Orders.idCustomer and Orders.idCategory = 'Notebooks'
group by Customers.Name