Having problems fully understanding GROUP BY - sql

Question

I'm going over some practise questions for an exam that I have coming up and I'm having a problem fully understanding group by. I see GROUP BY as the following: group the result set by one or more columns.
I have the following database schema
My query
SELECT orders.customer_numb, sum(order_lines.cost_line), customers.customer_first_name, customers.customer_last_name
FROM orders
INNER JOIN customers ON customers.customer_numb = orders.customer_numb
INNER JOIN order_lines ON order_lines.order_numb = orders.order_numb
GROUP BY orders.customer_numb, order_lines.cost_line, customers.customer_first_name, customers.customer_last_name
ORDER BY order_lines.cost_line DESC
What I'm struggling to understand
Why can't I simply use just GROUP BY orders.cost_line and group the data by cost_line?
What I'm trying to achieve
I'd like to achieve the name of the customer who has spent the most money. I just don't fully understand how to achieve this. I understand how joins work, I just can't seem to get my head around why I can't simply GROUP BY customer_numb and cost_line (with sum() used to calculate the amount spent). I seem to always get "not a GROUP BY expression", if someone could explain what I'm doing wrong (not just give me the answer), that would be great - I'd really appreciate that, and of course any resources that you have for using GROUP by properly.
Sorry for the long essay and If I've missed anything I apologise. Any help would be greatly appreciated.

Answer 1

I just can't seem to get my head around why I can't simply GROUP BY
customer_numb and cost_line (with sum() used to calculate the amount
spent).
When you say group by customer_numb you know that customer_numb uniquely identifies a row in the customer table (assuming customer_numb is either a primary or alternate key), so that any given customers.customer_numb will have one and only one value for customers.customer_first_name and customers.customer_last_name. But at parse time Oracle does not know, or at least acts like it does not know that. And it says, in a bit of panic, "What do I do if a single customer_numb has more than one value for customer_first_name?"
Roughly the rule is, expressions in the select clause can use expressions in the group by clause and/or use aggregate functions. (As well as constants and system variables that don't depend on the base tables, etc.) And by "use" I mean be the expression or part of the expression. So once you group on first name and last name, customer_first_name || customer_last_name would be a valid expression also.
When you have a table, like customers and are grouping by a primary key, or a column with a unique key and not null constraint, you can safely include them in group by clause. In this particular instance, group by customer.customer_numb, customer.customer_first_name, customer.customer_last_name.
Also note, that the order by in the first query will fail, since order_lines.cost_line doesn't have a single value for the group. You can order on sum(order_lines.cost_line) or use an column alias in the select clause and order on that alias
SELECT orders.customer_numb,
sum(order_lines.cost_line),
customers.customer_first_name,
customers.customer_last_name
FROM orders
INNER JOIN customers ON customers.customer_numb = orders.customer_numb
INNER JOIN order_lines ON order_lines.order_numb = orders.order_numb
GROUP BY orders.customer_numb,
customers.customer_first_name,
customers.customer_last_name
ORDER BY sum(order_lines.cost_line)
or
SELECT orders.customer_numb,
sum(order_lines.cost_line) as sum_cost_line,
. . .
ORDER BY sum_cost_line
Note: I've heard that some RDBMSes will imply additional expressions for the grouping without them being explicitly stated. Oracle is not one of those RDBMSes.
As for grouping by both customer_numb and cost_line Consider a DB with two customers, 1 and 2 with two orders of one line each:
Customer Number | Cost Line
1 | 20.00
1 | 20.00
2 | 35.00
2 | 30.00
select customer_number, cost_line, sum(cost_line)
FROM ...
group by customer_number, cost_line
order by sum(cost_line) desc
Customer Number | Cost Line | sum(cost_line)
1 | 20.00 | 40.00
2 | 35.00 | 35.00
2 | 30.00 | 30.00
The first row with highest sum(cost_line) is not the customer who spent the most.

Answer 2

I understand how joins work, I just can't seem to get my head around
why I can't simply GROUP BY customer_numb and cost_line (with sum()
used to calculate the amount spent).
This should give you the sum for every customer.
SELECT orders.customer_numb, sum(order_lines.cost_line)
FROM orders
INNER JOIN order_lines ON order_lines.order_numb = orders.order_numb
GROUP BY orders.customer_numb
Note that every column in the SELECT clause that's not an argument to an aggregate function is also a column in the GROUP BY clause.
Now you can join that with other tables to get more detail. Here's one way using a common table expression. (There are other ways to express what you want.)
with customer_sums as (
-- We give the columns useful aliases here.
SELECT orders.customer_numb as customer_numb,
sum(order_lines.cost_line) as total_orders
FROM orders
INNER JOIN order_lines ON order_lines.order_numb = orders.order_numb
GROUP BY orders.customer_numb
)
select c.customer_numb, c.customer_first_name, c.customer_last_name, cs.total_orders
from customers c
inner join customer_sums cs
on cs.customer_numb = c.customer_numb
order by cs.total_orders desc
Why can't I simply use just GROUP BY orders.cost_line and group the
data by cost_line?
Applying GROUP BY to order_lines.cost_line will give you one row for each distinct value in order_lines.cost_line. (The column orders.cost_line doesn't exist.) Here's what that data might look like.
OL.ORDER_NUMB OL.COST_LINE O.CUSTOMER_NUMB C.CUSTOMER_FIRST_NAME C.CUSTOMER_LAST_NAME
--
1 1.45 2014 Julio Savell
1 2.33 2014 Julio Savell
1 1.45 2014 Julio Savell
2 1.45 2014 Julio Savell
2 1.45 2014 Julio Savell
3 13.00 2014 Julio Savell
You can group by order_lines.cost_line, but it won't give you any useful information. This query
select order_lines.cost_line, orders.customer_numb
from order_lines
inner join orders on orders.customer_numb = order_lines.customer_numb
group by order_lines.cost_line;
should return something like this.
OL.COST_LINE O.CUSTOMER_NUMB
--
1.45 2014
2.33 2014
13.00 2014
Not terribly useful.
If you're interested in the sum of the order line items, you need to decide what column or columns to group (summarize) by. If you group (summarize) by order number, you'll get three rows. If you group (summarize) by customer number, you'll get one row.