Question

I am re-engineering code written in the SQL, and I got a use of cardinality in an order by clause. So on which basis it is going to order the records?

e.g.

ORDER BY
      cardinality(emp_ids),
      earliest_due_date

where emp_ids are prepared like this

 CAST(
       collect(employee.emp_id)
       AS
       NumberList
     ) AS emp_ids

Answer 1

It is documented that the return is numeric:

CARDINALITY returns the number of elements in a nested table. The
return type is NUMBER. If the nested table is empty, or is a null
collection, then CARDINALITY returns NULL.

So, your order by will return the emp_ids table with the least number of rows in it first (empty ones as it will return null will come last).