SUM function generates error in SQL query - php

Question

I'm trying to get information about an SQL table via a php script however, whenever I add a SUM function I get the following error "Notice: Trying to get property 'num_rows' of non-object"
I don't know what's going on but if anyone can please explain/point me in the right direction as to how to fix this problem that would be greatly appreciated
Here's the 'broken' code
<?php
require("connect.php");
$inNo = $_POST["inNo"];
$sql = "SELECT invoice.invoice_no, invoice.date, invoice.cust_id, invoice.emp_id, invoice_line.prod_id,
invoice_line.qty, product.cost_price, (product.cost_price * invoice_line.qty) AS `multi`,
customer.first_name AS `customer_fname`, customer.last_name AS `customer_lname`,
employee.first_name AS `emp_fname`, employee.last_name AS `emp_lname`,
product.name AS `proname`
FROM invoice INNER JOIN invoice_line ON invoice.invoice_no = invoice_line.invoice_no
INNER JOIN product ON invoice_line.prod_id = product.id
INNER JOIN customer ON invoice.cust_id = customer.id
INNER JOIN employee ON invoice.emp_id = employee.id, SUM(multi) AS `invoicetotal`
WHERE cust_id = '" . $inNo . "'";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
//open table
echo '<table class="table table-striped" id="outTable">';
echo "<tr><th>Product</th><th>Qty</th><th>Price</th><th>Total cost</th></tr>";
// output data of each row
$ctr = 0;
while($row = $result->fetch_assoc()) {
if ($ctr == 0)
{
echo "<h1>Invoice Number: " . $row["invoice_no"]. "</h1>";
echo "<p>Customer: " . $row["customer_fname"]. " " . $row["customer_lname"]."</p>";
echo "<p>Employee: " . $row["emp_fname"]. " " . $row["emp_lname"]. "</p>";
echo "<p>Date: " . $row["date"]. "</p>";
}
echo "
<tr>
<td>" . $row["proname"]. "</td>
<td>" . $row["qty"]. "</td>
<td>" . $row["cost_price"]. "</td>
<td>" . $row["multi"]. "</td>
</tr>";
$ctr++;
}
} else {
echo "0 results";
}
$conn->close();
?>

Answer 1

You need to insert the SUM() in the SELECT part above FROM:
SELECT
invoice.invoice_no,
invoice.date,
invoice.cust_id,
invoice.emp_id,
invoice_line.prod_id,
invoice_line.qty,
product.cost_price,
(product.cost_price * invoice_line.qty) AS `multi`,
customer.first_name AS `customer_fname`,
customer.last_name AS `customer_lname`,
employee.first_name AS `emp_fname`,
employee.last_name AS `emp_lname`,
product.name AS `proname`,
SUM(multi) AS `invoicetotal`
FROM invoice
INNER JOIN invoice_line
ON invoice.invoice_no = invoice_line.invoice_no
INNER JOIN product
ON invoice_line.prod_id = product.id
INNER JOIN customer
ON invoice.cust_id = customer.id
INNER JOIN employee
ON invoice.emp_id = employee.id` WHERE cust_id = 99
BTW, it's wise to format code - SQL code too - fine.