MySQL替换行，其中字段a等于x并且字段b等于y

Question

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?

Answer 1

Try replacing SET with VALUES . The syntax is:

REPLACE [LOW_PRIORITY | DELAYED]
  [INTO] tbl_name [(col_name,...)]
  {VALUES | VALUE} ({expr | DEFAULT},...),(...),...

Edit: Note that this only really works if you have set MemberID and TopicID as a unique key in your table. If you do this, then you should just be able to do either:

REPLACE INTO Views (MemberID, TopicID, Visited)
VALUES ('$_SESSION[SESS_MEMBER_ID]', '$threadid', '$t')

or

INSERT INTO Views (MemberID, TopicID, Visited)
VALUES ('$_SESSION[SESS_MEMBER_ID]', '$threadid', '$t')
ON DUPLICATE KEY UPDATE Visited = '$t'

(of course, you should be using proper placeholders so Little Bobby Tables doesn't make a visit and destroy your database)

Answer 2

I can't think of a way with only one query, but you could try this:

<?php
$where = "where `MemberID`=".$_SESSION['SESS_MEMBER_ID']." and `TopicID`=".$threadid;
if( mysql_fetch_assoc( mysql_query( "select * from `Views` ".$where)))
   mysql_query( "update `Views` set `Visited`='".$t."' ".$where);
else
   mysql_query( "insert into `Views` (`MemberID`, `TopicID`, `Visited`) values ('".$_SESSION['SESS_MEMBER_ID']."', '".$threadid."', '".$t."')");
?>

That would run two queries total.

Answer 3

If you're using the TIMESTAMP data type, it will update itself automatically every time the row is altered. So just call update on the row (if it exists, otherwise create it) and change just the topicId. The database will handle changing the time.

Really, I wouldn't use REPLACE. Instead, maybe you could create a stored procedure that checks to see if the row exists (just pass the memberId and topicId) and creates it if not, and then returns the timestamp to you. This makes more sense.

And using a stored proc means only one trip to the DB.