Month-Year-wise product sales query giving different results on query optimization

I want to retrieve sum(Quantity) of products ordered by month-year wise. For example, the output looks like:

product1........20......july-2008

product2........10......july-2008

Below is my default query, which seems to be giving correct results:

SELECT ITEMNAME, SUM(QTY), DATE_FORMAT(BILLDATE,'%M-%Y') FROM BILLITEMS
GROUP BY BILLDATE, ITEMNAME
ORDER BY YEAR(BILLDATE), MONTH(BILLDATE), ITEMNAME;

My concern is query speed. I tried following optimizations, but it is giving different results:

SELECT ITEM开发者_如何学CNAME, SUM(QTY), DATE_FORMAT(BILLDATE,'%M-%Y') FROM BILLITEMS
GROUP BY BILLDATE, ITEMNAME
ORDER BY DATE_FORMAT(BILLDATE,'%Y-%M'), ITEMNAME;

and another one:

SELECT ITEMNAME, SUM(QTY), DATE_FORMAT(BILLDATE,'%M-%Y') FROM BILLITEMS
GROUP BY DATE_FORMAT(BILLDATE,'%M-%Y'), ITEMNAME
ORDER BY DATE_FORMAT(BILLDATE,'%Y-%M'), ITEMNAME;