I have tried this, but no luck:
array('fn_export_purchased_amount', '#key'),
'linked' => false,
'export_only' => true
);
function fn_export_purchased_amount($product_id, $order_id)
{
return db_get_field(“SELECT amount FROM ?:order_details WHERE product_id = ?i AND order_id = ?i”, $product_id, $order_id);
}
return $schema;