Select random row for each group in a postgres table

Question

I have a table that is roughly:

id | category | link | caption | image

My goal is to fetch a random row from each distinct category in the table, for all the categories in the table. The plan is to then assign each row to a variable for its respective category.

Right now I'm using multiple SELECT statements resembling:

SELECT link, caption, image FROM table WHERE category='whatever' ORDER BY RANDOM() LIMIT 1

But this seems inelegant and creates more trips to the DB, which is expensive.

I'm pretty sure there's a way to do this with window functions in Postgres, but I have no experience with them and I'm not entirely sure how to use one to get what I want.

Thanks for any help!

Answer 1

Try something like:

SELECT DISTINCT ON (category) *
FROM table 
ORDER BY category, random();

Or with window functions:

SELECT * 
FROM (
SELECT *, row_number() OVER (PARTITION BY category ORDER BY random()) as rn
FROM table ) sub
WHERE rn = 1;