Learning something new every day (GROUP BY without an aggregation function)

I'm sure that we've all written procedures that send a report to all customers that ordered today (it doesn't have to be customers; it can be vendors or users in service calls or similar). I've always done this by means of a cursor such as

SELECT CUSTOMERS.CUSTNAME, COUNT (*)
FROM ORDERS, CUSTOMERS
WHERE ORDERS.CUST = CUSTOMERS.CUST
AND ORDERS.CURDATE = SQL.DATE8
GROUP BY 1;
 

I saw that someone had done something similar, but instead of using COUNT, he was using DISTINCT and instead of CUSTOMERS.CUSTNAME, a conditional expression was used, something like

SELECT DISTINCT (ORDERITEMS.ICURRENCY <> ORDERS.CURRENCY ? 
ORDERITEMS.ICURRENCY : ORDERS.CURRENCY),
...

I try to avoid using DISTINCT like the plague; to me it always smacks of lazy programming. One use that I do condone is if I want to know how many separate customers ordered today, as opposed to knowing how many orders each customer made today. I think that the above doesn't work properly because the field that DISTINCT is trying to filter is a conditional field whose value probably isn't known when DISTINCT does its work.

Today I discovered a new twist on the first query shown above: it turns out that the count is unnecessary (assuming of course that I only want a list of individual customers who ordered today). The following gives the desired result and should be faster as there is no need for counting. To my surprise, this syntax works in Priority.

SELECT CUSTOMERS.CUSTNAME
FROM ORDERS, CUSTOMERS
WHERE ORDERS.CUST = CUSTOMERS.CUST
AND ORDERS.CURDATE = SQL.DATE8
GROUP BY 1;

In other words, one can use 'GROUP BY' without an aggregation function (but not the other way around!).