Question

I know this is a common question, because I’ve asked it before, and have looked at several more, but still, despite basing my query on this, I cannot get it to work. see: Mysql join based on max(timestamp) and mysql LEFT join for right table max value

Basically I have a tables of different items (‘chairs’, ‘tables’) and each type has record with their own id for specific chairs or tables. Then I have a location table which keeps all the locations that have ever been, including the current one. The current location is noted by the MAX timestamp.

My query is meant to get all the items, and join to the current location. I have followed what others have said works, but it isn’t working for me. Any help is appreciated.

EDIT: I didn’t say how it fails: It gets records and makes the join, and it returns the max timestamp, but not the record info that corresponds with the MAX(timestamp) record. So it gives the right timestamp, but the floor and etc are NOT from the record with the MAX timestamp.

Thanks!

The query:

$q = "SELECT
        'chairs' AS work_type,
        chairs.id AS work_id,
        chairs.title,
        chairs.dimensions,
        CurrentLocations.floor,
        CurrentLocations.bin,
        CurrentLocations.bay,
        CurrentLocations.other
       FROM chairs
             LEFT JOIN ( 
                        SELECT 
                              locations.id AS loc_id,
                              locations.work_type AS clwt,
                              locations.work_id AS clwi,
                              locations.floor,
                              locations.bin,
                              locations.bay,
                              locations.other,
                              MAX(locations.timestamp) AS latestLocation
                            FROM
                               locations
                            GROUP BY
                               locations.work_type, locations.work_id
                            ) CurrentLocations
                ON CurrentLocations.clwi = chairs.id AND CurrentLocations.clwt = 'chairs'
        WHERE cur_loc LIKE '%19th Street%'
        ORDER BY 
            FIELD(floor, 'Basement', '3rd Floor', '4th Floor', '5th Floor'), 
            bin, 
            bay,
            other";

Answer 1

Try this:

    SELECT
        'chairs' AS work_type,        -- do you really need this in the result?
        c.id AS work_id,
        c.title,
        c.dimensions,
        l.floor,
        l.bin,
        l.bay,
        l.other
    FROM 
        chairs AS c
      LEFT JOIN
        ( SELECT work_id,
                 MAX(timestamp) AS latestLocation
          FROM   locations
          WHERE  work_type = 'chairs'
          GROUP BY work_id
        ) AS cl                                  -- CurrentLocations
          ON  cl.work_id = c.id
      LEFT JOIN
        locations AS l 
          ON  l.work_type = 'chairs'
          AND l.work_id = cl.work_id
          AND l.timestamp = cl.latestLocation 
    WHERE 
        c.cur_loc LIKE '%19th Street%'
    ORDER BY 
        FIELD(l.floor, 'Basement', '3rd Floor', '4th Floor', '5th Floor'), 
        l.bin, 
        l.bay,
        l.other ;

---

Description:

You want:

Basically I have a tables of different items (chairs, tables) and each type has record with their own id for specific chairs or tables. Then I have a locations table which keeps all the locations that have ever been, including the current one. The current location is noted by the maximum timestamp.

My query is meant to get all the items (chairs), and join to the current location.

So, for every item (that is a chair) you want only the latest location. For every work_id in table location, you want the MAX(timestamp). This can be easily found with a GROUP BY, as you already found out:

        ( SELECT work_id,
                 MAX(timestamp) AS latestLocation
          FROM   locations
          GROUP BY work_type
                 , work_id
        ) AS cl

But you also want to use other columns from that table and if you simply add them in the select list, it shows wrong results. (Note that in most DBMS, in order to use columns that are not in the GROUP BY list, you’d have to apply an aggregation function, like you applied MAX() to timestamp. MySQL allows this behaviour which if properly used, can have some benefits. But it hasn’t implemented it 100% failproof and thus applying it in some cases, like this one, can give wrong results. Postgres has a much better implementation of this feature that it won’t allow erroneous results.)

So, what you have to do is use the above GROUP BY query as a derived table (you name it cl or CurrentLocation, whatever you want) and join it to the locations table, using all columns from the GROUP BY list and the aggregated one (timestamp):

      JOIN
        locations AS l 
          ON  l.work_type = cl.work_type
          AND l.work_id = cl.work_id
          AND l.timestamp = cl.latestLocation 

After that, you can join them to the chairs table as usual. Since, you had a chairs LEFT JOIN locations in the original query, I kept these in the answer, too.

Another minor modification was that since you had CurrentLocations.work_id = 'chairs', you wanted only those rows from the locations table. So, it’s rather redundant to do first the grouping and then reject all rows that don’t match that condition. It’s probably more efficient to first apply the condition and then group by. The final grouping query and the join conditions are modified accordingly.

Also note that an index on (work_type, work_id, timestamp) will improve efficiency of the query (the cl subquery will be processed using only that index and the JOIN locations will use the index, too, to locate the rows needed from the locations table.)