Question

I have an Informix database containing measured temperature values for quite a few different locations. The measurements are taken every 15 min for all locations and then loaded with a timestamp into the same table. Table looks like this:

 locId    dtg               temp aaa      2009-02-25 10:00  15 bbb      2009-02-25 10:00  20 ccc      2009-02-25 10:00  24 aaa      2009-02-25 09:45  13 ccc      2009-02-25 09:45  16 bbb      2009-02-25 09:45  18 ddd      2009-02-25 09:45  12 aaa      2009-02-25 09:30  11 ccc      2009-02-25 09:30  14 bbb      2009-02-25 09:30  15 ddd      2009-02-25 09:30  10 

Now I would like a query that present me with the change in temperature between the two last measurements for all stations. And also, only the ones that has an updated measurement. For example in the table above, location ddd would not be included. So the result becomes:

 locId    change aaa      2 bbb      2 ccc      8 

I have tried alot but I can´t find any good solution. In reality it is about 700 locations that is asked from a web page so I think the query needs to be fairly efficient.

Would really appreciate some help!
//Jesper

Answer 1

set now = select max(dtg) from table; set then = select max(dtg) from table where dtg < now;  select locID, old.temp-new.temp from        table as old join       table as new        on old.locId = new.locID where       old.dtg = then and       new.dtg = now; 

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assumes that all times will be exact