title: Laravel SUM of multiple fields return null with raw query
body: <p>The following is my query : </p>
  
  <pre><code> $sales = DB::table('sales')
          -&gt;leftJoin('category_sales', 'category_sales.sale_id', '=', 'sales.id')
          -&gt;leftJoin('department_sales', 'department_sales.sale_id', '=', 'sales.id')
          -&gt;leftJoin('store_configs', 'store_configs.id', '=', 'sales.store_config_id')
          -&gt;select('sales.date',
              DB::raw('store_configs.store_dba'),
              DB::raw('sales.id'),
              DB::raw('(sales.taxable + sales.non_taxable + category_sales.amount + department_sales.amount) as total_sales'),
              DB::raw('0.0825*(sales.taxable + category_sales.amount + department_sales.amount) as total_tax'))
          -&gt;groupBy('date')-&gt;orderBy('date', 'desc')
          -&gt;get();
  </code></pre>
  
  <p>I get the right value when I have values on <code>category_sales and department_sales</code> table. Lets say, I do not have any <code>amount value for the sales_id in category_sales</code> table, the final result for <code>total_sales</code> and <code>total_tax</code> is null. </p>
  
  <p>My question is : how would I still sum the values of fields if the data is present ?</p>
  
  <pre><code>taxable, non_taxable, and amount'
  </code></pre>
  
  <p>in <code>category_sales and department_sales</code>
  are integer with <code>defaults</code> to <code>0</code></p>
  
  <p>My table structure just for an idea and is similar with department_sales: </p>
  
  <pre><code>CREATE TABLE `category_sales` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `amount` int(11) NOT NULL DEFAULT '0',
  `category_id` int(10) unsigned DEFAULT NULL,
  `sale_id` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `category_sales_category_id_index` (`category_id`),
  KEY `category_sales_sale_id_index` (`sale_id`),
  CONSTRAINT `category_sales_category_id_foreign` FOREIGN KEY (`category_id`) REFERENCES `categories` (`id`) ON DELETE CASCADE,
  CONSTRAINT `category_sales_sale_id_foreign` FOREIGN KEY (`sale_id`) REFERENCES `sales` (`id`) ON DELETE CASCADE) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
  </code></pre>