mysql query - error showing mysql results using inner join - php

Question

I have 2 tables that I want to merge. I want to print all the products with their corresponding total quantity and total amount.
This is what I have.
//Product Table
productID productName
1 A
2 B
3 C
//Order Record (This came from 2 tables that I have successfully merged)
orderID productID quantity amount
1 1 5 100
2 2 2 50
3 2 3 150
I want to do this
productID productName totalQuantity totalAmount
1 A 8 250
2 B 2 50
3 C 0 0
//instead of 0 for total Quantity and total Amount, it shows 2 and 50 respectively.
Here is my php code. It correctly outputs the the data for the first 2 rows (product A and B) but when it comes to the last row (product C), it copies the data for product B. Please tell me what's wrong in my code? Thank you in advance.
$products = $wpdb->get_results("SELECT * FROM wp_products");
foreach($products as $product){
$productID = $product->productID;
$productName = $product->productName;
$orders = $wpdb->get_results("SELECT a.productID, SUM(a.quantity) as totalQuantity, SUM(a.amount) as totalSales FROM a INNER JOIN b ON a.orderID = b.orderID GROUP BY productID");
if(is_null($orders)){
$totalQuantity = 0;
$totalSales = '0.00';
}
foreach($orders as $order){
$totalQuantity = $order->totalQuantity;
$totalSales = $order->totalSales;
}
$orderItem = array(
'productID' => $productID,
'productName' => $productName,
'totalQuantity' => $totalQuantity,
'totalSales' => $totalSales
);
$records[] = $orderItem;
}

Answer 1

Quick fix is (just add WHERE to your query):
$orders = $wpdb->get_results("SELECT
a.productID,
SUM(a.quantity) as totalQuantity,
SUM(a.amount) as totalSales
FROM a
INNER JOIN b
ON a.orderID = b.orderID
WHERE a.productID = $productID
GROUP BY productID");
But looking at your fragment, I believe you can simplify it (replace full fragment) to:
$records = $wpdb->get_results("SELECT
p.productID,
p.productName,
COALESCE(SUM(a.quantity),0) as totalQuantity,
COALESCE(SUM(a.amount),0) as totalSales
FROM wp_products p
LEFT JOIN a
GROUP BY p.productID");

Answer 2

I don't believe your query is correct.
I don't see the table names. Try changing it to:
FROM `tablename` AS a INNER JOIN `othertable` AS b
Swap out tablename and other table with the names for table a and b.