SQL: Group by minimum value in one field while selecting distinct rows

How about something like:

SELECT mt.*     
FROM MyTable mt INNER JOIN
    (
        SELECT id, MIN(record_date) AS MinDate
        FROM MyTable
        GROUP BY id
    ) t ON mt.id = t.id AND mt.record_date = t.MinDate

This gets the minimum date per ID, and then gets the values based on those values. The only time you would have duplicates is if there are duplicate minimum record_dates for the same ID.

I could get to your expected result just by doing this in mysql:

 SELECT id, min(record_date), other_cols 
  FROM mytable
  GROUP BY id

Does this work for you?

To get the cheapest product in each category, you use the MIN() function in a correlated subquery as follows:

    SELECT categoryid,
       productid,
       productName,
       unitprice 
    FROM products a WHERE unitprice = (
                SELECT MIN(unitprice)
                FROM products b
                WHERE b.categoryid = a.categoryid)

The outer query scans all rows in the products table and returns the products that have unit prices match with the lowest price in each category returned by the correlated subquery.