Oracle: getting maximum value of a group?
I'd tend to use analytic functions
SELECT monitor_id, host_name, calibration_date, value FROM (SELECT b.monitor_id, b.host_name, a.calibration_date, a.value, rank() over (partition by b.monitor_id order by a.calibration_date desc) rnk FROM table_name a, table_name2 b WHERE a.some_key = b.some_key) WHERE rnk = 1
You could also use correlated subqueries though that will be less efficient
SELECT monitor_id, calibration_date, value FROM table_name a WHERE a.calibration_date = (SELECT MAX(b.calibration_date) FROM table_name b WHERE a.monitor_id = b.monitor_id)
My personal preference is this:
SELECT DISTINCT monitor_id ,MAX(calibration_date) OVER (PARTITION BY monitor_id) AS latest_calibration_date ,FIRST_VALUE(value) OVER (PARTITION BY monitor_id ORDER BY calibration_date DESC) AS latest_valueFROM mytable;
A variation would be to use the
FIRST_VALUE syntax for
latest_calibration_date as well. Either way works.
The window functions solution should be the most efficient and result in only one table or index scan. The one I am posting here i think wins some points for being intuitive and easy to understand. I tested on SQL server and it performed 2nd to window functions, resulting in two index scans.
SELECT T1.monitor_id, T1.calibration_date, T1.valueFROM someTable AS T1WHERE NOT EXISTS ( SELECT * FROM someTable AS T2 WHERE T2.monitor_id = T1.monitor_id AND T2.value > T1.value) GROUP BY T1.monitor_id, T1.calibration_date, T1.value
And just for the heck of it, here's another one along the same lines, but less performing (63% cost vs 37%) than the other (again in sql server). This one uses a Left Outer Join in the execution plan where as the first one uses an Anti-Semi Merge Join:
SELECT T1.monitor_id, T1.calibration_date, T1.valueFROM someTable AS T1LEFT JOIN someTable AS T2 ON T2.monitor_id = T1.monitor_id AND T2.value > T1.valueWHERE T2.monitor_id IS NULLGROUP BY T1.monitor_id, T1.calibration_date, T1.value