It is known that GROUP BY produces one row per group. I want to produce multiple rows per group. The particular use case is, for example, selecting two cheapest offerings for each item.
It is trivial for two or three elements in the group:
select type, variety, price
from fruits
where price = (select min(price) from fruits as f where f.type = fruits.type)
or price = (select min(price) from fruits as f where f.type = fruits.type
and price > (select min(price) from fruits as f2 where f2.type = fruits.type));
(Select n rows per group in mysql)
But I am looking for a query that can show n rows per group, where n is arbitrarily large. In other words, a query that displays 5 rows per group should be convertible to a query that displays 7 rows per group by just replacing some constants in it.
I am not constrained to any DBMS, so I am interested in any solution that runs on any DBMS. It is fine if it uses some non-standard syntax.
For any database that supports analytic functions\ window functions, this is relatively easy
select *
from (select type,
variety,
price,
rank() over ([partition by something]
order by price) rnk
from fruits) rank_subquery
where rnk <= 3
If you omit the [partition by something], you'll get the top three overall rows. If you want the top three for each type, you'd partition by type in your rank() function.
Depending on how you want to handle ties, you may want to use dense_rank() or row_number() rather than rank(). If two rows tie for first, using rank, the next row would have a rnk of 3 while it would have a rnk of 2 with dense_rank. In both cases, both tied rows would have a rnk of 1. row_number would arbitrarily give one of the two tied rows a rnk of 1 and the other a rnk of 2.