Subtract quantity from another table

[Speed Ack]

I am making a simple Point of Sale system for personal use and I have two table namely: sales and items. Items to be sold are based on the list of items in the items table. Below is the insert code to the sales table.

if(isset($_POST['sales']))
      {
        $id        =    $_SESSION['posid'];
 
            $sql    = "    INSERT
                        INTO sales (uid, itemname, category, price, quantity, amount, paid, changeP, cashier, salesDate)
                        VALUES        (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)";
 
            if($stmt = mysqli_prepare($con, $sql))
                {
                mysqli_stmt_bind_param($stmt, "isssssssss", $id, $itemname, $category, $price, $quantity, $amount, $paid, $changeP, $cashier, $salesDate);
 
                $itemname    = mysqli_real_escape_string($con, $_POST['itemname']);
                $category    = mysqli_real_escape_string($con, $_POST['category']);
                $price        = mysqli_real_escape_string($con, $_POST['price']);
                $quantity    = mysqli_real_escape_string($con, $_POST['quantity']);
                $amount        = mysqli_real_escape_string($con, $_POST['amount']);
                $paid        = mysqli_real_escape_string($con, $_POST['paid']);
                $changeP    = mysqli_real_escape_string($con, $_POST['changeP']);
                $cashier      = mysqli_real_escape_string($con, $_POST['cashier']);
                $salesDate  = mysqli_real_escape_string($con, $_POST['salesDate']);
 
              if(mysqli_stmt_execute($stmt))
                {
                    echo "<script type='text/javascript'>alert('Payment was successfully processed.');
                            </script>";
                    echo "<script>window.location.href='pos.php'</script>";
                } 
                    else
                    {
                        echo "<script type='text/javascript'>alert('Oops! Something went wrong. Please try again.');
                                </script>";
                        echo "<script>window.location.href='pos.php'</script>";
                    }
 
                }
 
            mysqli_stmt_close($stmt);    // Close statement
 
    mysqli_close($con);        // Close connection
 }
 

Now, I want as I insert a purchased item in the sales table, the quantity in the items table to reduce by the number of items being purchased.
I really don't know how to go round this, please help.

[bakertaylor28]

After writing to the sales table, You need to store quantity of items to be reduced as a php variable, and then make a separate SQL call to the items table using UPDATE:

UPDATE sales 
SET columnname = foo;
$quantity = foo;
UPDATE items
SET columnname = $quantity;