Question

I have two queries which are returning different results when I would expect them to return the same results.

The first query returns the correct result.
The second returns a result, but it is incorrect.

Why is this and how can I fix the second statement so that it returns the same result? I have to use the HAVING clause in this statement.

1.

    SELECT
        CAST(CONCAT(DATE(`mytable`.`starttime`),' ',HOUR(`mytable`.`starttime`),':',LPAD(60*(MINUTE(`mytable`.`starttime`) DIV 60),2,'0'),':00') AS DATETIME) AS `date`,
        `mytable`.`id`
    FROM
        `mytable`
    WHERE
        `mytable`.`starttime`>='2011-07-01 00:00:00'
        AND `mytable`.`starttime`<='2011-07-01 23:59:59'
        AND `id` BETWEEN 1 AND 100
    GROUP BY
        `mytable`.`id`

2.

    SELECT
        CAST(CONCAT(DATE(`mytable`.`starttime`),' ',HOUR(`mytable`.`starttime`),':',LPAD(60*(MINUTE(`mytable`.`starttime`) DIV 60),2,'0'),':00') AS DATETIME) AS `date`,
        `mytable`.`id`
      FROM
        `mytable`
    WHERE 
        `id` BETWEEN 1 AND 100
    GROUP BY
        `mytable`.`id`
    HAVING `date` IN ('2011-07-01 00:00:00', '2011-07-01 01:00:00', '2011-07-01 02:00:00', '2011-07-01 03:00:00', '2011-07-01 04:00:00', '2011-07-01 05:00:00', '2011-07-01 06:00:00', '2011-07-01 07:00:00', '2011-07-01 08:00:00', '2011-07-01 09:00:00', '2011-07-01 10:00:00', '2011-07-01 11:00:00', '2011-07-01 12:00:00', '2011-07-01 13:00:00', '2011-07-01 14:00:00', '2011-07-01 15:00:00', '2011-07-01 16:00:00', '2011-07-01 17:00:00', '2011-07-01 18:00:00', '2011-07-01 19:00:00', '2011-07-01 20:00:00', '2011-07-01 21:00:00', '2011-07-01 22:00:00', '2011-07-01 23:00:00')

Thanks in advance for any help you can offer.

Answer 1

WHERE clause is applied before GROUPing while HAVING is applied after. So in your second query where you have GROUP BY with no WHERE clause MySql returns a random (undetermined) single row and then applies the HAVING clause to it.