Question

I have the following data.

--------------------------------------------------------------------------------
id             | value_name    | last_modified | system_id     | value_data    |
--------------------------------------------------------------------------------
1              | name1         | 2012-06-04    | 1             | aaa           |
2              | name1         | 2012-02-04    | 1             | bbb           |
3              | name1         | 2012-04-25    | 1             | ccc           |
4              | name2         | 2012-07-04    | 1             | ddd           |
5              | name2         | 2012-06-14    | 1             | eee           |
6              | name3         | 2011-09-05    | 1             | qqq           |
--------------------------------------------------------------------------------

Now I need to get only most recently modified values.

update

For the table above I want to get the following result

--------------------------------------------------------------------------------
id             | value_name    | last_modified | system_id     | value_data    |
--------------------------------------------------------------------------------
1              | name1         | 2012-06-04    | 1             | aaa           |
4              | name2         | 2012-07-04    | 1             | ddd           |
6              | name3         | 2011-09-05    | 1             | qqq           |
--------------------------------------------------------------------------------

I searched a lot here and in google as well, but the solutions I was able to find doesn’t actually work. For example one of the proposed solutions is usign join.

SELECT
    v.[id],
    v.[system_id],
    v.[value_name],
    v.[value_data]
FROM
    [values] v INNER JOIN
    (
        SELECT
            v.[id],      
            MAX(v.[last_modified]) AS last_modified
        FROM
            [values] v
        WHERE
            v.[system_id] = 1
        GROUP BY
            v.[id]
    ) s 
ON
    v.[id] = s.[id]

But sub-query is grouped by id (which is unique), so it will not calculate max last modification dates.
I’m using SQLite and simple query like that works for me

SELECT
    v.[id],
    v.[system_id],
    v.[value_name],
    v.[value_data],
    MAX(v.[last_modified]) AS last_modified
FROM
    [values] v
WHERE
    v.[system_id] = 1
GROUP BY
    v.[value_name]

But I remember from Oracle that you can’t select fields that are not listed in either aggregate functions or in group by statement, so I’m not sure this is guaranteed to give me expected result. Any suggestions how to solve that?

Thanks in advance.

Answer 1

If last_modified is unique per value_name, you can use this:

SELECT
    v.[id],
    v.[system_id],
    v.[value_name]
FROM
    [values] v INNER JOIN
    (
        SELECT
            v.[value_name],      
            MAX(v.[last_modified]) AS last_modified
        FROM
            [values] v
        WHERE
            v.[system_id] = 1
        GROUP BY
            v.[value_name]
    ) s 
ON
    v.[value_name] = s.[value_name]
AND
    v.[last_modified] = s.[last_modified]

If not, you will need to extract last id for last_modified grouped by value_name:

SELECT
    v.[id],
    v.[system_id],
    v.[value_name]
FROM
    [values] v INNER JOIN
    (
        SELECT max(id) ID
          FROM [values] v_max
         INNER JOIN
         (
             SELECT
                 [value_name],      
                 MAX([last_modified]) AS last_modified
             FROM
                 [values]
             WHERE
                 [system_id] = 1
             GROUP BY
                 [value_name]
         ) s 
          ON
              v_max.[value_name] = s.[value_name]
          AND
              v_max.[last_modified] = s.[last_modified]
         GROUP BY s.value_name, s.last_modified
   ) maxID
   ON v.ID = maxID.ID

Disclaimer: not tested.