Removing duplicate rows from relation - postgresql

Question

I have the following code which produces a relation:
SELECT book_id, shipments.customer_id
FROM shipments
LEFT JOIN editions ON (shipments.isbn = editions.isbn)
LEFT JOIN customers ON (shipments.customer_id = customers.customer_id)
In this relation, there are customer_ids as well as book_ids of books they have bought. My goal is to create a relation with each book in it and then how many unique customers bought it. I assume one way to achieve this is to eliminate all duplicate rows in the relation and then counting the instances of each book_id.
So my question is: How can I delete all duplicate rows from this relation?
Thanks!
EDIT: So what I mean is that I want all the rows in the relation to be unique. If there are three identical rows for example, two of them should be removed.

Answer 1

This will give you all the {customer,edition} pairs for which an order exists:
SELECT *
FROM customers c
JOIN editions e ON (
SELECT * FROM shipments s
WHERE s.isbn = e.isbn
AND s.customer_id = c.customer_id
);

Answer 2

The duplicates are in table shipments. You can remove these with a DISTINCT clause and then count them in an outer query GROUP BY isbn:
SELECT isbn, count(customer_id) AS unique_buyers
FROM (
SELECT DISTINCT isbn, customer_id FROM shipments) book_buyer
GROUP BY isbn;
If you want a list of all books, even where no purchases were made, you should LEFT JOIN the above to the list of all books:
SELECT isbn, coalesce(unique_buyers, 0) AS books_sold_to_unique_buyers
FROM editions
LEFT JOIN (
SELECT isbn, count(customer_id) AS unique_buyers
FROM (
SELECT DISTINCT isbn, customer_id FROM shipments) book_buyer
GROUP BY isbn) books_bought USING (isbn)
ORDER BY isbn;
You can write this more succinctly by joining before counting:
SELECT isbn, count(customer_id) AS books_sold_to_unique_buyers
FROM editions
LEFT JOIN (
SELECT DISTINCT isbn, customer_id FROM shipments) book_buyer USING (isbn)
GROUP BY isbn
ORDER BY isbn;