It's perfectly possible.
Let's assume your table structure looks like this:
CREATE TABLE [dbo].[Ratings](
[Evaluator] varchar(10),
[Evaluatee] varchar(10),
[Rating] int,
[Date] datetime
);
and the values like this:
INSERT INTO Ratings
SELECT 'Person 1', 'Person 2', 5, '2011-02-01' UNION
SELECT 'Person 1', 'Person 2', 2, '2011-03-01' UNION
SELECT 'Person 2', 'Person 1', 6, '2011-02-01' UNION
SELECT 'Person 2', 'Person 1', 3, '2011-03-01' UNION
SELECT 'Person 3', 'Person 1', 5, '2011-05-01'
Then the average rating for Person 1 is:
SELECT AVG(Rating) FROM Ratings r1
WHERE Evaluatee='Person 1' and not exists
(SELECT 1 FROM Ratings r2
WHERE r1.Evaluatee = r2.Evaluatee AND
r1.evaluator=r2.evaluator AND
r1.date < r2.date)
Result:
4
Or for all Evaluatee's, grouped by Evaluatee:
SELECT Evaluatee, AVG(Rating) FROM Ratings r1
WHERE not exists
(SELECT 1 FROM Ratings r2
WHERE r1.Evaluatee = r2.Evaluatee AND
r1.evaluator = r2.evaluator AND
r1.date < r2.date)
GROUP BY Evaluatee
Result:
Person 1 4
Person 2 2
This might look like it has an implicit assumption that no entries exist with the same date; but that's actually not a problem: If such entries can exist, then you can not decide which of these was made later anyway; you could only choose randomly between them. Like shown here, they are both included and averaged - which might be the best solution you can get for that border case (although it slightly favors that person, giving him two votes).
To avoid this problem altogether, you could simply make Date part of the primary key or a unique index - the obvious primary key choice here being the columns (Evaluator, Evaluatee, Date).
