preparing the query

// Get a database object
$db = JFactory::getDBO();

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

of course using the Database

Basic Query Execution

- query (This should be user after Set Query if you want to use getNumRows)

Query Execution Information

- getAffectedRows
-explain
-insertid

Insert Query Execution
-insertObject

Single Value Result

-loadResult

Single Row Results

-loadObject
-loadRow
-loadResultArray

Multi-Row Results

-loadObjectList
-loadRowList
-loadAssocList

Misc Result Set Methods

-getNumRows

a nice code would go like

$database = JFactory::getDBO();

$user_name=$_POST['username'];

$query = "SELECT * FROM #__users";

$database->setQuery($query);

$existing_users=$database->loadResultArray();

if (in_array($user_name, $existing_users)) {.. do something }