How can insert value to specific row in database

Question

i have PHP file that post data to database sequentially ( row by row ) using url , i want to know how can post data at specific row and if there is data in that row replace it by new value

<?php

$dbusername = "root"; 
$dbpassword = "root";  
$server = "192.168.137.150"; 
$dbconnect = mysql_pconnect($server, $dbusername, $dbpassword);
$dbselect = mysql_select_db("test",$dbconnect);
$sql = "INSERT INTO test.a0 (a0) VALUES ('".$_GET["value"]."')";    
mysql_query($sql)

?>

Answer 1

Depending on logic you need in the table, usually you have to look for the entry with SELECT first and then UPDATE if exists or INSERT if missing (2 queries required: SELECT + UPDATE/INSERT).

You can also make that field key and directly use INSERT ... ON DUPLICATE KEY UPDATE in a single query.