LOL, three late nights in a row, lose that last post o' mine.



Shawn Green wrote:
--- Peter Brawley <> wrote:


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
this is an old question.

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

item_id | location | status | time_stamp

As the items move new time stamped entries are added to the
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
the entry for an item when a location update is made because we
to keep the history.  We plan on removing items after a suitable
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;




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):

Shawn Green
Database Administrator

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