SQL: Get records having min/max value per group
Here’s one possible solution for a case when you need to fetch a set of records unique on one (or several) column with a min/max value in the other column (usually a timestamp). The solution doesn’t require any changes to the DB scheme (denormalization or creation of views) and can be performed in the scope of a single SQL query.
Scenario
Imagine you have a table named customer_subscriptions
with the following fields:
user_name
(we’ll use name instead ofuser_id
for simplicity here consideruser_name
to be unique for this example)plan_name
(we’ll use name instead ofplan_id
here as well)created_at
(the timestamp when the subscription was created)
For illustration, consider the following data in customer_subscriptions
:
Let’s say we want to show a table showing what was the first subscription plan for each user:
Subquery JOIN solution
- First, we must find the minimum
created_at
value for each user subscription entry separately in a subquery. - Then, we join the subquery on
user_name
andcreated_at
to filter down to the first subscription record for each user.
SELECT cs.user_name, cs.plan_name, cs.created_at
FROM customer_subscriptions cs
JOIN (
SELECT user_name, MIN(created_at) AS min_created_at
FROM customer_subscriptions
GROUP BY user_name
) AS sub
ON cs.user_name = sub.user_name AND cs.created_at = sub.min_created_at;
This solution would work perfectly for both MySQL and PostgreSQL.
PostgreSQL-specific solution
In PostgreSQL, we can use DISTINCT ON
to filter the resulting set to be unique on specific columns, such as user_name
. We won’t need to join an additional subquery.
DISTINCT ON (user_name)
: Ensures that eachuser_name
appears only once in the result set.ORDER BY user_name, created_at
: Orders the records byuser_name
and then bycreated_at
within each user_name, so the first record (with the smallestcreated_a
t value) is selected.
SELECT DISTINCT ON (user_name) user_name, plan_name, created_at
FROM customer_subscriptions
ORDER BY user_name, created_at;
Conclusion
Fetching the first subscription record for each user based on the earliest created_at
value can be efficiently achieved using different SQL constructs available in MySQL and PostgreSQL. While MySQL relies on subqueries and joins, PostgreSQL’s DISTINCT ON
clause provides a straightforward and elegant solution.