Hello list members.

I was trying to use the library and here is one thing, I didn't figure how to use it. Will be thankful, if you could help. Here is my example (not working):

(defun number-of-goods-you-have-the-most? ()
  (clsql:select [item_id] [as [count [*]] "num"]
        :from [table-name]
        :where
        [is [null [sale_date]]]
        :group-by [item_id]
        :order-by '(("num" :desc))
        :limit 1))

the non-working parts are [as [count [*]] "num"] and, possibly, :order-by '(("num" :desc)). What I was trying to do, should be quite obvious from the function name, yet, I'll explain just to make sure.

The table contains field item_id (which is not unique), it also has a field sale_date, which stores the date the item was sold. So, if the item was not sold (no sale_date) I would like to count the number of that item kind and return the number of items of the same kind, which are found the most in the table. Consider the example below:

item_id | sale_date
---------------------------
1 | NULL
1 | 2000-01-10
2 | NULL
3 | 2000-01-01
3 | NULL
1 | NULL
3 | 2000-01-01

The function then would return 2 because there are 2 items of item_id '1', which have not been sold yet.

Thanks in advance!