Accessing the Database by JDatabase in Joomla

For Accessing Database in Joomla cms User this object  " $db = JFactory::getDBO();".The getDBO function usually automatic create database connection.please see the below example .

For database connection use :
 $db = JFactory::getDBO();

 For Execute sql or mysql query : 
setQuery($query) 
Example :

$query = "SELECT * FROM #__example_table WHERE id = 999999;";
$db->setQuery($query);

Fetch result from database use :

loadAssoc(): loadAssoc() returns an associated array from a single record in the table.

Example :

$db->setQuery($query);
$row = $db->loadAssoc();
print_r($row);

loadResult() : Use loadResult() when you expect just a single value back from your database query.

Example :

$db = JFactory::getDBO();
$query = "
  SELECT COUNT(*)
    FROM ".$db->nameQuote('#__my_table')."
    WHERE ".$db->nameQuote('name')." = ".$db->quote($value).";
  ";
$db->setQuery($query);
$count = $db->loadResult();

loadRow(): loadRow() returns an indexed array from a single record in the table.

Example :

$db->setQuery($query);
$row = $db->loadRow();
print_r($row);

loadObject(): loadObject returns a PHP object from a single record in the table.

Example :

$db->setQuery($query);
$result = $db->loadObject();
print_r($result);

loadResultArray() : loadResultArray() returns an indexed array from a single column in the table.

Example :

$query = "
  SELECT name, email, username
    FROM . . . ";
. . .
$db->setQuery($query);
$column= $db->loadResultArray();
print_r($column);