The objective is to display the most recent transaction by custmer_id - postgresql

Question

My code ranks the last customer transaction by row number as planned, but I cannot filter my join to display on the last transaction per customer. The objective is to display the last detailed customer transaction per customer_id. I attempted to use the window function and then filter the resulting column.
CREATE TABLE customer1 (
customer_id INT PRIMARY KEY,
first_name VARCHAR(255),
last_name VARCHAR(255),
email VARCHAR(255),
created_at TIMESTAMP WITH TIME ZONE NOT NULL
);
CREATE TABLE purchase (
purchase_id INT PRIMARY KEY,
purchase_time TIMESTAMP WITH TIME ZONE NOT NULL,
customer_id INT NOT NULL,
FOREIGN KEY (customer_id) REFERENCES customer1(customer_id)
);
CREATE TABLE purchase_item (
purchase_item_id INT PRIMARY KEY,
purchase_id INT NOT NULL,
sku VARCHAR(255),
quantity INT NOT NULL,
total_amount_paid DECIMAL(10,2) NOT NULL,
FOREIGN KEY (purchase_id) REFERENCES purchase(purchase_id)
);
INSERT INTO customer1 (customer_id, first_name, last_name, email, created_at) VALUES
(1, 'James', 'Smith', 'jamessmith#example.com', clock_timestamp()),
(2, 'Mary', 'Johnson', 'maryjohnson#example.com', clock_timestamp()),
(3, 'John', 'Williams', 'johnwilliams#example.com', clock_timestamp()),
(4, 'Patricia', 'Brown', 'patriciabrown#example.com', clock_timestamp()),
(5, 'Michael', 'Garcia', 'michaelgarcia#example.com', clock_timestamp());
INSERT INTO purchase (purchase_id, purchase_time, customer_id) VALUES
(100, clock_timestamp(), 1),
(101, clock_timestamp(), 1),
(102, clock_timestamp(), 1),
(103, clock_timestamp(), 2),
(104, clock_timestamp(), 3),
(105, clock_timestamp(), 5);
INSERT INTO purchase_item(purchase_item_id, purchase_id, sku, quantity, total_amount_paid) VALUES
(200, 100, 'shoe_blk_42', 3, 300),
(201, 100, 'shoe_lace_white', 3, 2.5),
(202, 101, 'shorts', 1, 40),
(203, 102, 'bike', 1, 1995),
(204, 103, 'bike', 2, 3990),
(205, 103, 'shoe_wht_39', 2, 200),
(206, 104, 'shirt', 1, 60),
(207, 105, 'headphones', 1, 400);
SELECT DISTINCT customer1.customer_id,
first_name,
last_name,
email,
purchase.purchase_id,
purchase.purchase_time,
purchase_item.quantity,
purchase_item.total_amount_paid,
ROW_NUMBER()OVER (
PARTITION BY purchase.customer_id
ORDER BY
purchase.purchase_time DESC
) As order_queue
FROM customer1
JOIN purchase ON customer1.customer_id = purchase.customer_id
JOIN purchase_item ON purchase.purchase_id = purchase_item.purchase_id
WHERE order_queue = 1;

Answer 1

You can use DISTINCT ON to solve this:
select distinct on (customer1.customer_id)
customer1.customer_id,
first_name,
last_name,
email,
purchase.purchase_id,
purchase.purchase_time,
purchase_item.quantity,
purchase_item.total_amount_paid
FROM customer1
LEFT JOIN purchase ON customer1.customer_id = purchase.customer_id
LEFT JOIN purchase_item ON purchase.purchase_id = purchase_item.purchase_id
ORDER BY customer1.customer_id, purchase_time desc;
customer_id | first_name | last_name | email | purchase_id | purchase_time | quantity | total_amount_paid
-------------+------------+-----------+---------------------------+-------------+-------------------------------+----------+-------------------
1 | James | Smith | jamessmith#example.com | 102 | 2019-06-14 20:17:26.759086+00 | 1 | 1995.00
2 | Mary | Johnson | maryjohnson#example.com | 103 | 2019-06-14 20:17:26.759098+00 | 2 | 200.00
3 | John | Williams | johnwilliams#example.com | 104 | 2019-06-14 20:17:26.759109+00 | 1 | 60.00
4 | Patricia | Brown | patriciabrown#example.com | | | |
5 | Michael | Garcia | michaelgarcia#example.com | 105 | 2019-06-14 20:17:26.75912+00 | 1 | 400.00
(5 rows)
You can change the LEFT JOINs to JOINS if you don't want to see customers with no purchases.