Joining 2 tables with a timestamp of YEAR for the first item purchased by customer

Question

I have 2 tables "items" and "customers".

In both tables, "customer_id" is present and a single customer can have more than 1 item. In the "items" table there is also a timestamp field called "date_created" when an item was purchased.

I want to construct a query that can return each customer_id and item_id associated with the first item each customer bought in a specific year, let's say 2020.

My approach was

SELECT customer_id, items
INNER JOIN items ON items.customer_id=customers.customer_id

and then try to use the EXTRACT function to take care of the first item each customer bought in 2020 but I can't seem to extract the first item only for the specific year. I would really appreciate some help. I am using PostgreSQL. Thank you.

Answer 1

Just use distinct on:

select distinct on (customer_id) i.*
from items i
where date_created >= date '2020-01-01' and
      date_created < date '2021-01-01'
order by customer_id, date_created;

First, note the use of direct date comparisons. This makes it easier for the optimizer to choose the best execution plan.

distinct on is a handy Postgres extension that returns the first row encountered for the keys in parentheses, which must be the first keys in the order by. "First" is based on the subsequent order by keys.