👀 Views: 1 💬 Answers: 1 📅 Created: 2025-06-11

postgresql sql distinct SQL

Does anyone know how to I can't seem to get I'm encountering an unexpected behavior when using `DISTINCT ON` with `ORDER BY` in my PostgreSQL query... The goal is to return unique records based on a specific column while ensuring the results are ordered according to another criterion. However, I'm not getting the expected results. For instance, I have the following tables: ```sql CREATE TABLE orders ( id SERIAL PRIMARY KEY, customer_id INT, order_date TIMESTAMP, total DECIMAL ); CREATE TABLE customers ( id SERIAL PRIMARY KEY, name VARCHAR(100) ); INSERT INTO customers (name) VALUES ('Alice'), ('Bob'); INSERT INTO orders (customer_id, order_date, total) VALUES (1, '2023-01-01 10:00:00', 100.00), (1, '2023-01-02 10:00:00', 150.00), (2, '2023-01-01 09:00:00', 200.00); ``` I wrote this query to fetch the most recent order for each customer: ```sql SELECT DISTINCT ON (c.id) c.id, c.name, o.order_date, o.total FROM customers c JOIN orders o ON o.customer_id = c.id ORDER BY c.id, o.order_date DESC; ``` The expected output is: - Alice, 2023-01-02 10:00:00, 150.00 - Bob, 2023-01-01 09:00:00, 200.00 However, I'm seeing duplicate customer entries. Specifically, for Alice, the query returns: - Alice, 2023-01-01 10:00:00, 100.00 - Alice, 2023-01-02 10:00:00, 150.00 I've checked that I am using PostgreSQL version 14.2. I suspect I might be misunderstanding how `DISTINCT ON` works in conjunction with `ORDER BY`. Is there something I’m missing here? I also tried modifying the query to include `ORDER BY o.order_date DESC, c.id` but that didn't resolve the issue. Any insights or solutions would be greatly appreciated! I've been using Sql for about a year now. Any ideas how to fix this? This is my first time working with Sql 3.9. Any pointers in the right direction? I'm open to any suggestions.