What is the primary key? I'll assume the key consists of (ts,
username), in which case the folllowing should do what you want:

SELECT ts, username, ... /* other columns */
FROM YourTable AS T
WHERE ts =
(SELECT MAX(ts)
FROM YourTable
WHERE username = T.username)

It really helps if you include DDL with questions like this (basically
a CREATE TABLE statement, including keys and constraints). The exact
table structure may make a big difference to the possible solutions.
The usual recommendation that you shouldn't use SELECT * in production
code also applies.

--
David Portas
SQL Server MVP
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