Understand Sub-Queries - sql

Question

I was initially looking to see a breakdown of the total dollar business that each vendor has done (indirectly via the distributor) with each customer, where I'm trying not to use the Inner Join Syntax and used the Query below for this purpose:
select customers.cust_id, Vendors.vend_id, sum(quantity*item_price) as total_business from
(((Vendors left outer join Products
on Products.vend_id = Vendors.vend_id)
left outer join OrderItems --No inner joins allowed
on OrderItems.prod_id = Products.prod_id)
left outer join Orders
on Orders.order_num = OrderItems.order_num)
left outer join Customers
on Customers.cust_id = Orders.cust_id
where Customers.cust_id is not null -- THE ONLY DIFFERENCE BETWEEN QUERY1 AND QUERY2
group by Customers.cust_id, Vendors.vend_id
order by total_business
Now, I am trying to see the query output results for all vendor-customer combinations, including those combinations where there was no business transacted and am trying to write this via a single SQL Query. My teacher provided this solution, but I honestly cannot understand the logic at all, as I've never come across Sub-queries.
select
customers.cust_id,
Vendors.vend_id,
sum(OrderItems.quantity*orderitems.item_price)
from
(
customers
inner join
Vendors on 1 = 1
)
left outer join --synthetic product using joins
(
orders
join
orderitems on orders.order_num = OrderItems.order_num
join
Products on orderitems.prod_id = products.prod_id
) on
Vendors.vend_id = Products.vend_id and
customers.cust_id = orders.cust_id
group by customers.cust_id, vendors.vend_id
order by customers.cust_id
Thanks a lot

Answer 1

I would write this query as:
select c.cust_id, v.vend_id, coalesce(cv.total, 0)
fro Customers c cross join
Vendors v left outer join
(select o.cust_id, v.vend_id, sum(oi.quantity * oi.item_price) as total
from orders o join
orderitems oi
on o.order_num = oi.order_num join
Products p
on oi.prod_id = p.prod_id
group by o.cust_id, v.vend_id
) cv
on cv.vend_id = v.vend_id and
cv.cust_id = c.cust_id
order by c.cust_id;
The structure is quite similar. Both version start by creating a cross product between all customers and vendors. This creates all the rows in the output result set. Next, the aggregation needs to be calculated at this level. In the above query, this is done explicitly as a subquery which aggregates the values to the customer/vendor level. (In the original query, this is done in the outer query.)
The final step is joining these together.
Your teacher should be encouraging you to use table aliases, particularly table abbreviations. You should also be encouraged to use the proper join. So, although you can express a cross join as an inner join with on 1=1, a cross join is part of the SQL language, not a hack.
Similarly, parentheses in the from clause can make the logic harder to follow. Explicit subqueries are more easily read.