Shawn,

Yep that's the theory, but where (i) the aggregate result is a column value, rather than a sum or average for example, and (ii) id is unique, I have not been able to get MySQL to give a wrong <second_col> value with that approach, eg try the following with the northwind database (it ought to be doable in one query, but this machine's version of the MySQL server crashed on that):

-- 'wrong' max, omitting nulls
DROP TABLE IF EXISTS o1;
CREATE TABLE o1
SELECT orderid,shipcity,MAX(shippeddate) AS latest
FROM orders
WHERE shippeddate IS NOT NULL
GROUP BY orderid;

-- correct max, again omitting nulls
DROP TABLE IF EXISTS o2;
CREATE TABLE o2
SELECT
  orderid,
  shipcity,
  (SELECT MAX(shippeddate) AS latest FROM orders o2 WHERE o2.orderid=o1.orderid) AS latest
FROM orders o1
GROUP BY orderid
HAVING latest IS NOT NULL;

-- report o1 and o2 rows which do not match:
SELECT MIN(TableName) as TableName, orderid, shipcity, latest
FROM (
  SELECT 'o1' AS TableName,orderid,shipcity,latest FROM o1
  UNION ALL
  SELECT 'o2' as TableName,orderid,shipcity,latest FROM o2
) AS tmp
GROUP BY orderid, shipcity,latest
HAVING COUNT(*) = 1;
Empty set (0.01 sec)

PB

-----

Shawn Green wrote:
--- Peter Brawley <peter.brawley@earthlink.net> wrote:

  
Brian
    
Hi,

I hope this is the right list for this question. If not, I'm happy
to get help on where to post this question.  Apologies in advance
      
if 
    
this is an old question.

We are designing a simple a tracking database with a table of
      
entries 
    
showing the current location of each item in the system.  Something
      
simple like this.

Table_xyz
item_id | location | status | time_stamp

As the items move new time stamped entries are added to the
      
database.  
    
How would you query to find the current location of all the items 
currently in the system.  As you might expect we don't want to
      
replace 
    
the entry for an item when a location update is made because we
      
need 
    
to keep the history.  We plan on removing items after a suitable
      
delay 
    
when they reach their destination.
      
An item is in the most recent location for that item_id, right? Then
...

SELECT item_id, location,MAX(timestamp)
FROM table_xyz
GROUP BY item_id;

PB
    
Thanks

-Brian


      
Peter, 

I am surprised at you ;-)  You should know that the query you sent
won't work like you said. Here's your query suggestion:

  
SELECT item_id, location,MAX(timestamp)
FROM table_xyz
GROUP BY item_id;
    

The item_id will be unique (thanks to the GROUP BY item_id) and the
MAX(timestamp) will also be correct (again because of the GROUP BY) but
the middle column, `location`, will not necessarily be the location
code of the record with the MAX(timestamp) :-(

Because that column is neither part of the GROUP BY clause or covered
by an aggregate function, the engine should throw an error. However,
MySQL tries to be nicer than that so it just picks a random value from
all of the rows where the item_id's are the same.  The only way to get
to the groupwize maximum (in this case the record with the latest date
from a group of records sharing the same ID) is by using one of the
techniques listed here (temp table, subquery, concat hack):

http://dev.mysql.com/doc/refman/4.1/en/example-maximum-column-group-row.html

Shawn Green
Database Administrator

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