nested select using count to tally each row in Postgres - postgresql

Question

I have a table where each row is a unique order with a unique order_id, but users can have multiple rows/orders.
Orders table -> order_id, user_id, orderedat (datetime), sales
I am trying to return a query that calculates, for each order_id, how many previous orders the associated user has made. (Essentially, does this row represent the user's first order? 5th order? 20th order? etc.)
I'm trying something like this nested select, but am getting an error "more than one row returned by a subquery used as an expression."
SELECT
order_id,
user_id,
COUNT (order_id) AS order_n
FROM
orders
WHERE orderedat >= (
SELECT
orderedat
FROM
fulfillments
GROUP BY
order_id
)
GROUP BY
order_id
Any thoughts on how to achieve this in postgres?
///////////////
Further complication: with another column called "Status," how to only count rows with specific values in Status? I'd like to just skip orders in the number unless they have a status of "paid" or "placed". For example:
data:
order_id user_id orderedat status
001 max 10/1/14 paid
002 max 10/20/14 placed
003 max 10/21/14 cancelled
004 bill 10/5/14 deleted
005 max 10/31/14 paid
006 bill 10/24/14 placed
results:
order_id user_id orderedat orders_so_far
001 max 10/1/14 1
002 max 10/20/14 2
003 max 10/21/14 null
005 max 10/31/14 3
004 bill 10/5/14 null
006 bill 10/24/14 1

Answer 1

This can be done using a window function:
SELECT order_id,
user_id,
orderdat,
row_number() over (partition by user_id order by orderedat ) as orders_so_far
FROM orders
order by user_id, orderdat