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 <[EMAIL PROTECTED]> wrote:BrianHi, 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 advanceifthis is an old question. We are designing a simple a tracking database with a table ofentriesshowing the current location of each item in the system. Somethingsimple like this. Table_xyz item_id | location | status | time_stamp As the items move new time stamped entries are added to thedatabase.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 toreplacethe entry for an item when a location update is made because weneedto keep the history. We plan on removing items after a suitabledelaywhen 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; PBThanks -BrianPeter, 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 __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com |
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