oracle - GROUP BY / aggregate function confusion in SQL

Question

I need a bit of help straightening out something, I know it's a very easy easy question but it's something that is slightly confusing me in SQL.

This SQL query throws a 'not a GROUP BY expression' error in Oracle. I understand why, as I know that once I group by an attribute of a tuple, I can no longer access any other attribute.

SELECT * 
FROM order_details 
GROUP BY order_no

However this one does work

SELECT SUM(order_price)
FROM order_details
GROUP BY order_no

Just to concrete my understanding on this.... Assuming that there are multiple tuples in order_details for each order that is made, once I group the tuples according to order_no, I can still access the order_price attribute for each individual tuple in the group, but only using an aggregate function?

In other words, aggregate functions when used in the SELECT clause are able to drill down into the group to see the 'hidden' attributes, where simply using 'SELECT order_no' will throw an error?

Answer 1

In standard SQL (but not MySQL), when you use GROUP BY, you must list all the result columns that are not aggregates in the GROUP BY clause. So, if order_details has 6 columns, then you must list all 6 columns (by name - you can't use * in the GROUP BY or ORDER BY clauses) in the GROUP BY clause.

You can also do:

SELECT order_no, SUM(order_price)
  FROM order_details
 GROUP BY order_no;

That will work because all the non-aggregate columns are listed in the GROUP BY clause.

You could do something like:

SELECT order_no, order_price, MAX(order_item)
  FROM order_details
 GROUP BY order_no, order_price;

This query isn't really meaningful (or most probably isn't meaningful), but it will 'work'. It will list each separate order number and order price combination, and will give the maximum order item (number) associated with that price. If all the items in an order have distinct prices, you'll end up with groups of one row each. OTOH, if there are several items in the order at the same price (say ￡0.99 each), then it will group those together and return the maximum order item number at that price. (I'm assuming the table has a primary key on (order_no, order_item) where the first item in the order has order_item = 1, the second item is 2, etc.)