SQL - Advanced "forensic" queries - sql

Question

I want to find out which one of the customers has purchased the largest total amount of items, with no date range
These are the columns I have:
customers.customerid
orders.orderid
orderdetails.productid
orderdetails.quantity
But I'm stuck here:
Start with customerid
Pull the orderid's from the customerid (using a join I assume)
Pull the quantity and individual productid's (another join? lol)
select * from orders returns 196 records.
select distinct customerid from orders returns 74 records, so that means 74 'distinct' customers placed orders
How to complete this?

Answer 1

First, find out how many items each customer ordered. To do that, you have to join the order detail and orders table:
select o.customerid, sum(od.quantity) as nbr_items
from orderdetail od
inner join orders o
on o.orderid = od.orderid
group by o.customerid
But if you need the customer name as well, which is fairly likely, you would have to join the customer table, too:
select o.customerid, sum(od.quantity) as nbr_items
from orderdetail od
inner join orders o
on o.orderid = od.orderid
inner join customer c
on c.customerid = o.customerid
group by o.customerid
But of course, you asked specifically for the customer with the MOST TOTAL AMOUNT of items, so sort descending by that total:
select o.customerid, sum(od.quantity) as nbr_items
from orderdetail od
inner join orders o
on o.orderid = od.orderid
inner join customer c
on c.customerid = o.customerid
group by o.customerid
order by 2 DESC
And use the TOP N function to get the first one only:
select top 1 o.customerid, sum(od.quantity) as nbr_items
from orderdetail od
inner join orders o
on o.orderid = od.orderid
inner join customer c
on c.customerid = o.customerid
group by o.customerid
order by 2 DESC