Select unique latest record from sql database

I have a log table like this:

username productID purchasetime
---------------------------------------------------
user1 12 01/02/1999
user1 12 03/05/2000
user2 23 02/03/2000
user2 31 02/04/2000
user2 23 02/04/2000
....

I want to select users that purchased product later than a predefined time, say 01/01/1998, but if a user purchase a same product at different dates, then only the latest one will be selected. How to write the query? Thanks a lot.

Code:

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Select UserName from table where purchasetime in (select max(purchasetime) from table grpup by username)

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Thanks..
But a new problem.
I have a separate table to keep Students' Address. It saves the DateGiven of that address. How can I Read the Latest Address of each student. I tried the following but error occours. Please help me.

SELECT Std_ID, Line1, Line2, City
FROM AddressBook
WHERE Std_ID IN (SELECT Std_ID, MAX(DateGiven) FROM AddressBook GROUP BY Std_ID)

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Msg 116, Level 16, State 1, Line 1
Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.
=======

Try:

Code:

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SELECT Std_ID, Line1, Line2, City
FROM AddressBook AB1
WHERE AB1.DateGiven
 IN (SELECT MAX(AB2.DateGiven)
    FROM AddressBook AB2
    WHERE AB2.Std_ID = AB1.Std_ID)

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If using SQL Server 2005 or higher, then the ROW_Number() can help you to avoid having to make the subqueries and MAX.
Just make something like
Row_Number() OVER (Partition By username Order By purchasetime DESC) AS RowNumber
and then select where RowNumber is 1.