GROUP BY gives wrong result with MIN() aggregate function

As a MySQL DBA, I sadly admit that MySQL can be rather cavalier in its SQL processing. One of the most infamous feats of this is its GROUP BY behavior.

As example, Aaron Bertrand answered the post Why do we use Group by 1 and Group by 1,2,3 in SQL query? where he described MySQL's GROUP BY as cowboy who-knows-what-will-happen grouping. I just had to agree.

SUGGESTION

Rewrite the GROUP BY using code

select code,min(price) as total 
from product group by code

Do three things

1. Make the query a subquery
2. Use price as alias instead on total
3. Join it back to the product table on code and price

Here is the proposed query

select b.* from
(select code,min(price) as price from product group by code) a
inner join product b using (code,price);

or

select b.* from
(select code,min(price) as price from product group by code) a
inner join product b ON a.code=b.code AND a.price=b.price;

Checkout the SQL Fiddle for this

GIVE IT A TRY !!!

UPDATE 2017-01-06 16:17 EST

If there exists more than 1 row with the same minimum price for a given code, you have take the query, make it a subquery, join it to retrieve the minimum id for each (code,price) and join that back to product by id:

select bb.* from
(select a.code,a.price,min(b.id) id from
(select code,min(price) as price from product group by code) a
inner join product b using (code,price)
group by a.code,a.price) aa
inner join product bb using (id);

Checkout the SQL Fiddle for that

That should do the trick:

SELECT
    p.*
FROM
    product p
    JOIN
    (
        SELECT
            code, min(price) AS min_price
        FROM
            product
        GROUP BY
            code
    ) m ON p.code = m.code AND p.price = m.min_price 
ORDER BY
    p.id ;

Caveat: if there are ties (i.e.: the min(price) appears in more than one row per group), all rows will be returned. If, in case of a tie, you want another behaviour, things get a bit more complicated... a second choice criterium is needed (if possible, one that cannot get another tie), and another level of subquerying.

You can check this query also @ SQLFiddle

You can check all @RolandoMySQLDBA explanations for all tne "non-standard" things that go behind a GROUP BY in mySQL. It can easily be tricky.