MySQL REPLACE rows WHERE field a equals x and field b equals y
I'm trying to construct a 'new comments' feature where the user is shown the number of new comments since the last visit.
I've constructed a 'Views' table that contains the topic id, the user id and a timestamp.  What's the best way of updating the timestamp every time the user visits that topic or inserting a fresh row if one doesn't exist?  At the moment I have:

    REPLACE INTO Views (MemberID, TopicID, Visited) SET ('$_SESSION[SESS_MEMBER_ID]', '$threadid', '$t') WHERE MemberID = $_SESSION[SESS_MEMBER_ID] AND TopicID = $threadid

However, this does not work.  Is it possible to do this in one query?
MySQL REPLACE rows WHERE field a equals x and field b equals y
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I'm trying to construct a 'new comments' feature where the user is shown the number of new comments since the last visit.
I've constructed a 'Views' table that contains the topic id, the user id and a timestamp.  What's the best way of updating the timestamp every time the user visits that topic or inserting a fresh row if one doesn't exist?  At the moment I have:

    REPLACE INTO Views (MemberID, TopicID, Visited) SET ('$_SESSION[SESS_MEMBER_ID]', '$threadid', '$t') WHERE MemberID = $_SESSION[SESS_MEMBER_ID] AND TopicID = $threadid

However, this does not work.  Is it possible to do this in one query?