php - is it possible creation of duplicate ip in this code? -

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i have popup rotator count unic user's ip per day(no duplicate ip allowed user in same date)

$userip = mysql_real_escape_string($_server['remote_addr']); $date = date('y-m-d'); $result = mysql_query("select `ip`  `popupip` ip = '$userip' , userid = $secid , date='$date'"); $num = mysql_num_rows($result);     if ($num > 0) {     // **it duplicate not insert in popupip table**     }else{     // **it not duplicate ip insert in popupip table**     }

above code example.i know full code.

but when phpmyadmin popupip table there few duplicate ip user (the exact same ip address user on same date)

how possible?

extra information: in popupip userid int(11) , date "date type 2014-05-30" , ip varchar. page may opens "as fast possible @ same time" popup pages. there relation between openning page fast @ same time user , duplicate ip creation? there bug in mysql? (maybe big bug!!!!)

yes, possible. it's classical case of race condition.

the quick explanation:

there chance 2 requests simultaneously pass first check, $num == 0 , both insert new row.

to eliminate need create unique constraint covers (user_id, ip, date) columns

the long explanation:


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