percent_rank vs cume_dist
It seems pandas `percent_rank` works like the `cume_dist` of SQL databases.

As `ibis-framework` tries to use the same pandas API as much as possible ... `ibis-framework` `percent_rank` is equal to `pandas`.

As in SQL databases there are these 2 operations, I need a way to implement the SQL `percent_rank`

I don't know very well which path I should take. Is there an initial thoughts here: https://github.com/ibis-project/ibis/issues/1975

I wonder if there was any discussion about this topic before.

Any comment, recommendation or guidance would be very appreciated.
