Group by groups of 3 by column - sql

Question

Say I have a table that looks like this:
| id | category_id | created_at |
| 1 | 3 | date... |
| 2 | 4 | date... |
| 3 | 1 | date... |
| 4 | 2 | date... |
| 5 | 5 | date... |
| 6 | 6 | date... |
And imagine there are a lot more entries. I'd like to grab these in a way that they are fresh, so ordering them by created_at DESC - but I'd also like to group them by category, in groups of 3!
So in pseudocode it looks something like this:
Go to category 1
-> Pick last 3
Go to category 2
-> Pick last 3
Go to category 3
-> Pick last 3
And so forth, starting over from category_id 1 when there's no other category to grab from. This will then be paginated as well so I need to make it work with offset & limit as well somehow.
I'm not at all sure where to start or what they keywords to google for are. I'd be happy with some nudges in the right direction so I can find the answer myself, or a full answer.

Answer 1

Another case for the window function row_number().
Just the latest 3 rows per category
SELECT id, category_id, created_at
FROM (
SELECT id, category_id, created_at
, row_number() OVER (PARTITION BY category_id
ORDER BY created_at DESC) AS rn
FROM tbl
) sub
WHERE rn < 4
ORDER BY category_id, rn;
The latest 3 rows per category, plus later rows
If you want to append the rest of the rows (your question gets fuzzy if and how):
SELECT *
FROM (
SELECT id, category_id, created_at
, row_number() OVER (PARTITION BY category_id
ORDER BY created_at DESC) AS rn
FROM tbl
) sub
ORDER BY (rn > 3), category_id, rn;
One can sort by the outcome of a boolean expression (rn > 3):
FALSE (0)
TRUE (1)
NULL (because default is NULLS LAST - not applicable here)
This way, the latest 3 rows per category come first and all the rest later.
Or use a CTE and UNION ALL:
WITH cte AS (
SELECT id, category_id, created_at
, row_number() OVER (PARTITION BY category_id
ORDER BY created_at DESC) AS rn
FROM tbl
)
)
SELECT id, category_id, created_at
FROM cte
WHERE rn < 4
ORDER BY category_id, rn
)
UNION ALL
)
SELECT id, category_id, created_at
FROM cte
WHERE rn >= 4
ORDER BY category_id, rn
);
Same result.
All parentheses required to attach ORDER BY in individual legs of a UNION query.