GROUP BY / aggregate function confusion in SQL

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.)

The order in which SQL is written is not the same order it is executed.

Normally, you would write SQL like this:

SELECT
FROM
JOIN
WHERE
GROUP BY
HAVING
ORDER BY

Under the hood, SQL is executed like this:

FROM
JOIN
WHERE
GROUP BY
HAVING
SELECT
ORDER BY

Reason why you need to put all the non-aggregate columns in SELECT to the GROUP BY is the top-down behaviour in programming. You cannot call something you have not declared yet.

Read more: https://sqlbolt.com/lesson/select_queries_order_of_execution

SELECT * 
FROM order_details 
GROUP BY order_no

In the above query you are selecting all the columns because of that its throwing an error not group by something like.. to avoid that you have to mention all the columns whichever in select statement all columns must be in group by clause..

 SELECT * 
    FROM order_details 
    GROUP BY order_no,order_details,etc

etc it means all the columns from order_details table.