如何使API PHP检查该值是否存在于另一个表中然后插入？

Question

I want to make API for android. I'm working on a form in php that inserts data to MySQL, but before the data is inserted there is a field that must be checked in another table before inserting. If this value exist in the other table, then the data is inserted in the main table, if not, then data is not inserted.

This Is my php

<?php

if($_SERVER['REQUEST_METHOD']=='POST') {

$response = array();

$username = $_POST['username'];
$SN = $_POST['SN'];


require_once('dbConnect.php');

$sql = "SELECT * FROM produk WHERE SN ='$SN'";
$check = mysqli_fetch_array(mysqli_query($con,$sql));
if(isset($check)){
 $response["value"] = 0;
 $sql = "INSERT INTO farm (username,SN) VALUES('$username','$SN')";
 $response["message"] = "Successful";
 echo json_encode($response);
} else {
   $response["value"] = 1;
   $response["message"] = "oops! Try Again";
   echo json_encode($response);

}

mysqli_close($con);
} else {
$response["value"] = 0;
$response["message"] = "oops! Try Again";
echo json_encode($response);
}

?>

How solve the code?

Answer 1

Try with below code, you need to put a condition to check the value exists in another table. Check mysqli_num_rows for the other table, if num_rows > 0 then insert into main table:

<?php

if($_SERVER['REQUEST_METHOD']=='POST') {

$response = array();

$username = $_POST['username'];
$SN = $_POST['SN'];


require_once('dbConnect.php');

$sql = "SELECT * FROM produk WHERE SN ='$SN'";
$check = mysqli_num_rows(mysqli_query($con,$sql));
if($check > 0){
 $response["value"] = 0;
 $sql = "INSERT INTO farm (username,SN) VALUES('$username','$SN')";
 $response["message"] = "Successful";
 echo json_encode($response);
} else {
   $response["value"] = 1;
   $response["message"] = "oops! Try Again";
   echo json_encode($response);

}

mysqli_close($con);
} else {
$response["value"] = 0;
$response["message"] = "oops! Try Again";
echo json_encode($response);
}

?>