How to call Direct SQL Queries and join to collection In Magento2

In you block or model files you need to initialize resource then you need to call connection

that is

protected $_resource;

and

public function __construct(
    \Magento\Backend\Block\Template\Context $context,
    \Magento\Framework\App\Resource $resource,
    array $data = []
) {
    $this->_resource = $resource;
    parent::__construct($context, $data);
}

for connection

protected function getConnection()
{
    if (!$this->connection) {
        $this->connection = $this->_resource->getConnection('core_write');
    }

    return $this->connection;
}

below is example in block file

<?php
/**[email protected]*/
namespace Sugarcode\Test\Block;

class Joinex extends \Magento\Framework\View\Element\Template
{
    protected $_coreRegistry = null;
    protected $_orderCollectionFactory = null;
    protected $connection;
    protected $_resource;

    public function __construct(
        \Magento\Backend\Block\Template\Context $context,
        \Magento\Framework\Registry $registry,
        \Magento\Framework\App\Resource $resource,
        \Magento\Sales\Model\Resource\Order\CollectionFactory $orderCollectionFactory,
        array $data = []
    ) {
        $this->_orderCollectionFactory = $orderCollectionFactory;
        $this->_coreRegistry = $registry;
        $this->_resource = $resource;
        parent::__construct($context, $data);
    }



    public function _prepareLayout()
    {
        return parent::_prepareLayout();
    }

    protected function getConnection()
    {
        if (!$this->connection) {
            $this->connection = $this->_resource->getConnection('core_write');
        }
        return $this->connection;
    }

    public function getDirectQuery()
    {
        $table=$this->_resource->getTableName('catalog_product_entity'); 
        $sku = $this->getConnection()->fetchRow('SELECT sku,entity_id FROM ' . $table);
        return $sku;
    }

    public function getJoinLeft()
    {
          $orders = $this->_orderCollectionFactory->create();
          $orders->getSelect()->joinLeft(
            ['oce' => 'customer_entity'],
            "main_table.customer_id = oce.entity_id",
            [   
                'CONCAT(oce.firstname," ", oce.lastname) as customer_name',
                'oce.firstname',
                'oce.lastname',
                'oce.email'
            ]
        );

        //$orders->getSelect()->__toString(); $orders->printlogquery(true); exit;
        return $orders; 
    }
}

you have use old call for beta version core_write and core_read in rc is like this :

 protected  _resource;
  public function __construct(Context $context,
\Magento\Framework\App\ResourceConnection $resource)
  {
    $this->_resource = $resource;
    parent::__construct($context);

  }

get adapter :

$connection = $this->_resource->getConnection(\Magento\Framework\App\ResourceConnection::DEFAULT_CONNECTION);

get table and select:

$tblSalesOrder = $connection->getTableName('sales_order');
$result1 = $connection->fetchAll('SELECT quote_id FROM `'.$tblSalesOrder.'` WHERE entity_id='.$orderId);

complete course from here

I have achieved this in following way. I have a custom file where I am creating object of it and it worked. Check it once.

class Sample extends \Magento\Framework\App\Http implements \Magento\Framework\AppInterface
{

    public function sampleMethod()
    {
        $connection = $this->_objectManager->create('\Magento\Framework\App\ResourceConnection');
        $conn = $connection->getConnection();
        $select = $conn->select()
            ->from(
                ['o' => 'catalog_category_entity_varchar']
            )
            ->where('o.value=?', '2');
        $data = $conn->fetchAll($select);
        print_r($data);
    }

}

Try and let me know if it works for you.