How do I: remove duplicates?

PRQL doesn't have a specific distinct keyword. Instead duplicate tuples in a relation can be removed by using group and take 1:

from employees
select department
group employees.* (
  take 1
)

This also works with a wildcard:

from employees
group employees.* (take 1)

Remove duplicates from each group?

To select a single row from each group group can be combined with sort and take:

# youngest employee from each department
from employees
group department (
  sort age
  take 1
)

Note that we can't always compile to DISTINCT; when the columns in the group aren't all the available columns, we need to use a window function:

from employees
group {first_name, last_name} (take 1)