Check if row exists in the database before inserting - php

Question

$DBH = new PDO($dsn, $username, $password, $opt);
$DBH->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$DBH->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$STH = $DBH->prepare("INSERT INTO requests (id,imdbid,msg) VALUES ('',:imdbid,:msg)");
$STH->bindParam(':imdbid', $_POST['imdbid']);
$STH->bindParam(':msg', $_POST['msg']);
$STH->execute();
echo "<p>Successfully Requested ".$_POST['imdbid']."! Thanks!</p>";
Is there either some SQL Query that will check and insert or what?
I need it to check if whatever the user typed is already in the db so if the user typed in a imdbid that is already there then it wont continue inserting anything. How would I do this? I know I can do a fetch_all and make a foreach for it but doesnt that only work after you execute?

Answer 1

It's better to set a constraint on your columns to prevent duplicate data instead of checking and inserting.
Just set a UNIQUE constraint on imdbid:
ALTER TABLE `requests` ADD UNIQUE `imdbid_unique`(`imdbid`);
The reason for doing this is so that you don't run into a race condition.
There's a small window between finishing the check, and actually inserting the data, and in that small window, data could be inserted that will conflict with the to-be-inserted data.
Solution? Use constraints and check $DBH->error() for insertion errors. If there are any errors, you know that there's a duplicate and you can notify your user then.
I noticed that you are using this, $DBH->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);. In this case, you don't need to check ->error() because PDO will throw an exception. Just wrap your execute with try and catch like this:
$duplicate = false;
try {
$STH->execute();
} catch (Exception $e) {
echo "<p>Failed to Request ".$_POST['imdbid']."!</p>";
$duplicate = true;
}
if (!$duplicate)
echo "<p>Successfully Requested ".$_POST['imdbid']."! Thanks!</p>";

Answer 2

Simply run a query prior to inserting.
If found die the script:
$DBH = new PDO($dsn, $username, $password, $opt);
$DBH->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$DBH->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$sql = 'SELECT COUNT(*) from requests WHERE imdbid = :imdbid';
$stmt = $DBH->prepare($sql);
$stmt->execute(array(':imdbid' => $_POST['imdbid']));
if($stmt->fetchColumn()){ die('Already exist');}
$STH = $DBH->prepare("INSERT INTO requests (id,imdbid,msg) VALUES ('',:imdbid,:msg)");
$STH->bindParam(':imdbid', $_POST['imdbid']);
$STH->bindParam(':msg', $_POST['msg']);
$STH->execute();
echo "<p>Successfully Requested ".$_POST['imdbid']."! Thanks!</p>";
or alternatively make the msg field unique.
Using a stored Procedure:
DELIMITER //
CREATE PROCEDURE insert_request_msg(IN `p_imbd`, IN `p_msg`)
IF NOT EXISTS (SELECT COUNT(*) from requests WHERE imdbid = p_imbd)
BEGIN
INSERT INTO requests (id,imdbid,msg) VALUES ('',p_imbd,p_msg)
END
END IF; //
DELIMITER ;
You call it in one query like this:
$STH = $DBH->prepare('
call insert_request_msg(:imdbid,:msg)
');
$STH->bindParam(':imdbid', $_POST['imdbid']);
$STH->bindParam(':msg', $_POST['msg']);

Answer 3

Try this
$DBH = new PDO($dsn, $username, $password, $opt);
$DBH->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$DBH->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$STH = $DBH->prepare("INSERT INTO requests (id,imdbid,msg) VALUES ('',:imdbid,:msg) WHERE NOT EXISTS(SELECT imdbid FROM requests WHERE imdbid =:imdbid)");
$STH->bindParam(':imdbid', $_POST['imdbid']);
$STH->bindParam(':msg', $_POST['msg']);
$STH->execute();
echo "<p>Successfully Requested ".$_POST['imdbid']."! Thanks!</p>";
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