Need a postgresql query to group and order - sql

Question

I have a customers table as following:
customername, ordername, amount
=============================
bob, book, 20
bob, computer, 40
steve,hat, 15
bill, book, 12
bill, computer, 3
steve, pencil, 10
bill, pen, 2
I want to run a query to get the following result:
customername, ordername, amount
=============================
bob, computer, 40
bob, book, 20
bob, ~total~, 60
steve, hat, 15
steve, pencil, 10
steve, ~total~,25
bill, book, 12
bill, computer, 3
bill, pen, 2
bill, ~total~, 17
I want the amount for each customer to be ordered from max to min and a new ordername as "~total~" (must always be the last row for each customer) with a result as sum of all amount for the same customer.
So, in above example, bob should be the first since the total=60, steve the second (total=25) and bill the third (total=17).

Answer 1

Use:
SELECT x.customername,
x.ordername,
x.amount
FROM (SELECT a.customername,
a.ordername,
a.amount,
y.rk,
1 AS sort
FROM CUSTOMERS a
JOIN (SELECT c.customername,
ROW_NUMBER() OVER (ORDER BY SUM(c.amount) DESC) AS rk
FROM CUSTOMERS c
GROUP BY c.customername) y ON y.customername = a.customername
UNION ALL
SELECT b.customername,
'~total~',
SUM(b.amount),
ROW_NUMBER() OVER (ORDER BY SUM(b.amount) DESC) AS rk,
2 AS sort
FROM CUSTOMERS b
GROUP BY b.customername) x
ORDER BY x.rk, x.customername, x.sort, x.amount DESC
You could look at using GROUP BY ROLLUP, but the ordername value would be NULL so you'd have to post-process it to get that replaced with "~total~"...