How to select most recent values? - sql

Question

I have a logging table collecting values from many probes:
CREATE TABLE [Log]
(
[LogID] int IDENTITY (1, 1) NOT NULL,
[Minute] datetime NOT NULL,
[ProbeID] int NOT NULL DEFAULT 0,
[Value] FLOAT(24) NOT NULL DEFAULT 0.0,
CONSTRAINT Log_PK PRIMARY KEY([LogID])
)
GO
CREATE INDEX [Minute_ProbeID_Value] ON [Log]([Minute], [ProbeID], [Value])
GO
Typically, each probe generates a value every minute or so. Some example output:
LogID Minute ProbeID Value
====== ================ ======= =====
873875 2014-07-27 09:36 1972 24.4
873876 2014-07-27 09:36 2001 29.7
873877 2014-07-27 09:36 3781 19.8
873878 2014-07-27 09:36 1963 25.6
873879 2014-07-27 09:36 2002 22.9
873880 2014-07-27 09:36 1959 -30.1
873881 2014-07-27 09:36 2005 20.7
873882 2014-07-27 09:36 1234 23.8
873883 2014-07-27 09:36 1970 19.9
873884 2014-07-27 09:36 1991 22.4
873885 2014-07-27 09:37 1958 1.7
873886 2014-07-27 09:37 1962 21.3
873887 2014-07-27 09:37 1020 23.1
873888 2014-07-27 09:38 1972 24.1
873889 2014-07-27 09:38 3781 20.1
873890 2014-07-27 09:38 2001 30
873891 2014-07-27 09:38 2002 23.4
873892 2014-07-27 09:38 1963 26
873893 2014-07-27 09:38 2005 20.8
873894 2014-07-27 09:38 1234 23.7
873895 2014-07-27 09:38 1970 19.8
873896 2014-07-27 09:38 1991 22.7
873897 2014-07-27 09:39 1958 1.4
873898 2014-07-27 09:39 1962 22.1
873899 2014-07-27 09:39 1020 23.1
What is the most efficient way to get just the latest reading for each Probe?
e.g.of desired output (note: the "Value" is not e.g. a Max() or an Avg()):
LogID Minute ProbeID Value
====== ================= ======= =====
873899 27-Jul-2014 09:39 1020 3.1
873894 27-Jul-2014 09:38 1234 23.7
873897 27-Jul-2014 09:39 1958 1.4
873880 27-Jul-2014 09:36 1959 -30.1
873898 27-Jul-2014 09:39 1962 22.1
873892 27-Jul-2014 09:38 1963 26
873895 27-Jul-2014 09:38 1970 19.8
873888 27-Jul-2014 09:38 1972 24.1
873896 27-Jul-2014 09:38 1991 22.7
873890 27-Jul-2014 09:38 2001 30
873891 27-Jul-2014 09:38 2002 23.4
873893 27-Jul-2014 09:38 2005 20.8
873889 27-Jul-2014 09:38 3781 20.1

Answer 1

This is another approach
select *
from log l
where minute =
(select max(x.minute) from log x where x.probeid = l.probeid)
You can compare the execution plan w/ a fiddle - http://sqlfiddle.com/#!3/1d3ff/3/0

Answer 2

Try this:
SELECT T1.*
FROM Log T1
INNER JOIN (SELECT Max(Minute) Minute,
ProbeID
FROM Log
GROUP BY ProbeID)T2
ON T1.ProbeID = T2.ProbeID
AND T1.Minute = T2.Minute
You can play around with it on SQL Fiddle

Answer 3

Your question is: "What is the most efficient way to get just the latest reading for each Probe?"
To really answer this question, you test to test different solutions. I would generally go with the row_number() method suggested by #jyparask. However, the following might have better performance:
select l.*
from log l
where not exists (select 1
from log l2
where l2.probeid = l.probeid and
l2.minute > l.minute
);
For performance, you want an index on log(probeid, minute).
Although not exactly your problem, here is an example of where not exists performs better than other methods on SQL Server.

Answer 4

;WITH MyCTE AS
(
SELECT LogID,
Minute,
ProbeID,
Value,
ROW_NUMBER() OVER(PARTITION BY ProbeID ORDER BY Minute DESC) AS rn
FROM LOG
)
SELECT LogID,
Minute,
ProbeID,
Value
FROM MyCTE
WHERE rn = 1