UPDATE查询更新以前使用[重复]设置的所有TIMEDATE列

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I am working on a membership script, and can't for the life of me figure out whats wrong with my query... anyone have any idea's? Think I need a second set of eyes.. Originally I was just sending

$sqlquery2 = "UPDATE users SET lastvisit = now() WHERE id = '" . $id ."'";
but it was updating the joined_date column as well. So I tried this and broke it further.

CODE EXCERPT :

            //These variables are pulled from prior query

            $id = $row['id'];
            $hashed_password = $row['password'];
            $username = $row['username'];
            $joined = $row['join_date'];
            $salt = $row['salt'];
            $email = $row['email'];

            //compare password pulled from database
            if(password_verify($password,$hashed_password)){
                $sqlquery2 = "UPDATE users SET lastvisit = now(), join_date = ".$joined." WHERE id = '" . $id ."'";

//$joined is equal to 2016-10-19 17:24:08

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Please check whether your joined_date is a TIMESTAMP column. If so following will help you. http://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization.html