PRQL
A tail function that would take the last n results
We could use something like
LIMIT n OFFSET ROW_COUNT-n
If we could get the ROW_COUNT for the last transformation , which I was thinking might be possible if we took the entire generated query and used it with a COUNT subquery to get ROW_COUNT
This was said on discord:
aljazerzen — 14/06/2022
DataFrame libraries usually do have head and tail, but I think this goes against the "only one way of doing things" so i would be better to refrain from adding so similar functions
max-sixty — 14/06/2022
I think it could still be useful — "reverse sort and use head" is not that easy for a user.
My main concern is that it might be difficult to implement, and it's unclear what to do when there's no sorting — would we just take head with the assumption that sorting isn't guaranteed?
I also agree that in SQL databases, order of unsorted table is not guaranteed, so we can just take head.
Regarding the implementation:
head is actually exactly the same as take
from employees
sort salary
tail 100
could be expanded to:
from employees
sort [-salary]
head 100
sort salary
which would translate to:
WITH (
SELECT * FROM employees
ORDER BY salary DESC -- inverse the sort here
LIMIT 100 -- take head
) table_0
SELECT table_0.* FROM table_0
ORDER BY salary ASC -- now correct the order
