1-example_query_1.sql

SELECT * FROM purchases ORDER BY created_at DESC;

2-example_query_1_results

  id  | customer_id | subtotal | shipping |   tax   |  total   |         created_at         
------+-------------+----------+----------+---------+----------+----------------------------
 1101 |       83922 |   102.44 |     4.99 |  8.7074 | 116.1374 | 2013-07-05 22:49:09.20922
 1051 |       83922 |    51.75 |     4.99 | 4.39875 | 61.13875 | 2013-07-05 22:48:45.353301
 1079 |       48582 |    47.02 |     4.99 |  3.9967 |  56.0067 | 2013-07-05 22:48:02.866348
 1057 |       48582 |    30.64 |     4.99 |     2.6 |    38.23 | 2013-07-05 22:47:12.535409
 1048 |       48582 |    81.72 |     4.99 |    6.95 |    93.66 | 2013-07-05 22:46:18.304818

3-example_query_2.sql

SELECT MAX(created_at) AS most_recent FROM purchases GROUP BY customer_id;

4-example_query_2_results

        most_recent         
----------------------------
 2013-07-05 22:48:02.866348
 2013-07-05 22:49:09.20922

5-row_number_query.sql

SELECT *, 
  ROW_NUMBER() OVER (
    PARTITION BY customer_id ORDER BY created_at DESC
  )
  AS created_order
FROM purchases

6-row_number_results

  id  | customer_id | subtotal | shipping |   tax   |  total   |         created_at         | created_order 
------+-------------+----------+----------+---------+----------+----------------------------+---------------
 1079 |       48582 |    47.02 |     4.99 |  3.9967 |  56.0067 | 2013-07-05 22:48:02.866348 |             1
 1057 |       48582 |    30.64 |     4.99 |     2.6 |    38.23 | 2013-07-05 22:47:12.535409 |             2
 1048 |       48582 |    81.72 |     4.99 |    6.95 |    93.66 | 2013-07-05 22:46:18.304818 |             3
 1101 |       83922 |   102.44 |     4.99 |  8.7074 | 116.1374 | 2013-07-05 22:49:09.20922  |             1
 1051 |       83922 |    51.75 |     4.99 | 4.39875 | 61.13875 | 2013-07-05 22:48:45.353301 |             2

7-final_query.sql

WITH ordered_purchases AS (
  SELECT *, 
    ROW_NUMBER() OVER (
      PARTITION BY customer_id ORDER BY created_at DESC
    )
    AS created_order
  FROM purchases
)
SELECT * FROM ordered_purchases WHERE created_order = 1;

7-final_query_results

  id  | customer_id | subtotal | shipping |  tax   |  total   |         created_at         | created_order 
------+-------------+----------+----------+--------+----------+----------------------------+---------------
 1079 |       48582 |    47.02 |     4.99 | 3.9967 |  56.0067 | 2013-07-05 22:48:02.866348 |             1
 1101 |       83922 |   102.44 |     4.99 | 8.7074 | 116.1374 | 2013-07-05 22:49:09.20922  |             1