Coding guidelines

• For SQL statement, when using GROUP_CONCAT it's highly recommended to use | as separator. Because by default, most database use comma as a separator. When raw data is used for csv output; column cannot contain comma. 
  For example, instead of 
  SELECT GROUP_CONCAT(order_id) -- which yield 3,5,7 -> breaks the CSV download
  we should do
  SLECT GROUP_CONCAT(order_id  SEPARATOR '|') -- which yields 3|5|7 -> does not break for the CSV download
  
  
• To check if a value exists, use if ($value !== null)
  Do not use if ($value)
  because when $value is zero, if ($value) does not pass.
  Example code:
  
          if($quote->getIsMultiShipping() && $quoteItem->getCustomPrice()) {
              $item->setPrice($quoteItem->getCustomPrice())->setBaseOriginalPrice($quoteItem->getCustomPrice());
  
  This code is wrong. When customPrice is zero (no-charge), this if statement does not pass.
  
  Rewrite it to this:
  
           if($quote->getIsMultiShipping() && ($quoteItem->getCustomPrice() !== null)) {
              $item->setPrice($quoteItem->getCustomPrice())->setBaseOriginalPrice($quoteItem->getCustomPrice());
  
  now, if customPrice is zero, this if statement passes