if specific row = null, execute query - mysql

Question

how do i check if the value of a column is null, and only then execute the query? for example:
col1 col2 col3
01 abc
i run a query which first checks if the record exists or not; if it exists, it should execute the update query and if it doesn't exist, it executes the insert query. how do i check if col3 is null and if it is null, it should execute the update query. .
$sql = "SELECT uid FROM `users` WHERE uid = '" . $user_id . "'";
$result = mysql_query($sql,$conn) or die('Error:' .mysql_error());
$totalrows = mysql_num_rows($result);
if($totalrows < 1)
{
insertUser($user_id,$sk, $conn);
}
else
{
updateSessionKey($user_id,$sk,$conn);
}

Answer 1

http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html
Not really checking a value of the column, but I don't think you actually need that.
You need to have uid as a UNIQUE column. You try to insert a row for a new user with the given uid; if it finds the user with the same uid, then you do the update instead.
UPDATE:
I guess you did not bother to read the link.
I did not test it, but it should be something like this:
INSERT INTO users (uid, name, session)
VALUES ('login', 'Real Name', 'SeSsIoN_iD')
ON DUPLICATE KEY UPDATE session='SeSsIoN_iD'
This will insert the user if he does not exist, and if he does, it will set a new session key. OR, if you want to preserve the old session key if he already has one,
INSERT INTO users (uid, name, session)
VALUES ('login', 'Real Name', 'SeSsIoN_iD')
ON DUPLICATE KEY UPDATE session=IFNULL(session, 'SeSsIoN_iD')
One query, not three. You were not already doing it.

Answer 2

$sql = "SELECT * FROM `users` WHERE uid = '" . $user_id . "'";
$result = mysql_query($sql,$conn) or die('Error:' .mysql_error());
$totalrows = mysql_num_rows($result);
if($totalrows < 1)
{
$res = mysql_fetch_array($sql);
if(!empty($res['col3'])) {
insertUser($user_id,$sk, $conn);
}
}
else
{
updateSessionKey($user_id,$sk,$conn);
}

Answer 3

Is this what you mean?
If the record does not exist -> insert.
If the record does exist and its col3 is null -> update
If the record does exist, but its col3 is not null -> do nothing?
That could be achieved like this (untested):
$sql = "SELECT uid, col3 FROM `users` WHERE uid = '" . $user_id . "'";
$result = mysql_query($sql,$conn) or die('Error:' .mysql_error());
$totalrows = mysql_num_rows($result);
if($totalrows < 1)
{
insertUser($user_id,$sk, $conn);
}
else
{
$col3value = mysql_result($result, 0, 'col3');
if (is_null($col3value))
{
updateSessionKey($user_id,$sk,$conn);
}
}