检查MySQL是否已存在数据；如果不存在，则检查INSERT。 [PHP + jQuery Ajax]

Question

I'm having trouble creating php code that would insert values into MySQL database but only if they don't already exist.

I send array from javascript to PHP file using $.ajax type POST.

Do I need additional 'SELECT' query to check if values already exist?

PHP File(Works, inserts values):

<?php
SESSION_START();
include('config.php');
if(isset($_POST['predictedMatches'])&&$_SESSION['userid']){
    $predictedMatches=$_POST['predictedMatches'];
    $userid=$_SESSION['userid'];
}else die("ERROR");
$sql="";
foreach($predictedMatches as $predictedMatch){
    $sql.="INSERT INTO predictions(result,userFK,matchFK,tournamentFK) VALUES('".$predictedMatch['result']."','".$userid."','".$predictedMatch['id']."','".$predictedMatch['tourid']."');";
}
    if($conn->multi_query($sql) === TRUE){
        echo "OK";
    }else{
        echo "Error: " . $sql . "<br>" . $conn->error;
    }
$conn->close();
?>

Answer 1

Use the ON DUPLICATE KEY UPDATE feature. It won't insert, if the primary key exists. But you have to update some value, so use the column which is in no index or in the least indexes () in your case probably result). Your primary key has to be composted out of the three FKs:

ALTER TABLE `predictions` ADD PRIMARY KEY( `userFK`, `matchFK`, `tournamentFK`);

PHP-Code, just the SQL statment (I'm a Java Guy, so i tried my best)

$sql.="INSERT INTO predictions (result, userFK, matchFK, tournamentFK) "
."VALUES('".$predictedMatch['result'] ."','".$userid."','"
.$predictedMatch['id']."','".$predictedMatch['tourid']."') "
."ON DUPLICATE KEY UPDATE result = result ;";

To know if the query was inserted you have to look at the affected row count:

• 1 Row - Insert
• 2 Rows - Update

Take a look at $conn->affected_rows after the query.

Performance

INSERT ... ON DUPLICATE KEY UPDATE is definitively faster than a SELECT and INSERT but it's slower than an INSERT of just the needed datasets. The update is done in the database, even if it is the same value. Unfortunately there is no ON DUPLICATE KEY UPDATE INGNORE . If you have a lot of inserts, that will result in updates, than it may be better to use a cache, lookup values in an array and compare with the array before inserting. Only use the ON DUPLICATE KEY UPDATE as fallback.