IF-ELSE using PHP and SQL - php

Question

I am creating a stock trading game with PHP and SQL. I have a 'buy' function that adds the stock id, user id and quantity to a table named 'ownedstocks'.
I am having trouble in implementing a simple 'If-Else' statement.
Basically, what I want:
If the user id and the stock id of the stock being purchased are already exists in 'ownedstocks' table, then I just want to update the quantity.
Else if no rows exist for the given user id and stock id, then I want to insert a new row.
I have the code but unsure of using IF-ELSE.
Thanks in advance!
<?php
include_once 'header.php';
$user = $_SESSION['user'];
$id = $_SESSION['id'];
$price = $_SESSION['price'];
$amount=$_POST['amount'];
$total = $amount*$price;
$balance = $_SESSION['balance'];
$con=mysqli_connect("localhost","root","usbw","stocktrading");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$newbalance = ($balance - $total);
queryMysql("UPDATE ownedstocks
SET quantity = (quantity+$amount)
WHERE ownedstocks.userid = '$user' and ownedstocks.stockid = '$id'");
queryMysql("INSERT INTO ownedstocks
VALUES ('$user', '$id', '$amount')");
queryMysql("UPDATE members
SET balance = $newbalance
WHERE members.user = '$user'");
echo("You just purchased $id costing $price per stock<br/><br/>
You bought $amount stocks<br/><br/>
To calculate your bill: $price x $amount which equals $total<br/><br/>
You have just spent $total, your new balance is $newbalance <br/><br/>");
?>

Answer 1

What you want is the MySQL INSERT INTO ON DUPLICATE KEY UPDATE function.
Basically:
INSERT INTO ownedstocks values (...) ON DUPLICATE KEY UPDATE amount = amount + '$amount'

Answer 2

Add unique on (user, stock_id):
ALTER TABLE `ownedstocks` ADD UNIQUE (
`user` ,
`stock_id`
);
Then do something like:
INSERT INTO ownedstocks
VALUES ('$user', '$id', '$amount')
ON DUPLICATE KEY UPDATE
`amount` = `amount` + '$amount';

Answer 3

You could use the mysqli.affected-rows variable http://php.net/manual/en/mysqli.affected-rows.php to achieve the IF statement.