this article is taken from :

http://simplythebest.net/scripts/ajax/ajax_password_strength.html

To install the script on your Web page follow these instructions:

1) Download this ZIP file with jQuery 1.2.1 and jquery.pstrength-min.1.2.js. Unpack the files and upload both to your server.

2) Insert this anywhere before the password field on the page:

<script type=”text/javascript” src=”js/jquery.js”></script> // adjust the path as is required
<script type=”text/javascript” src=”js/jquery.pstrength-min.1.2.js”>
</script>
<script type=”text/javascript”>
$(function() {
$(‘.password’).pstrength();
});
</script>

3) Insert class=”password” into your input box tag to identify a password box:

<INPUT class=”password” type=password name=”Password”>

4) Insert these tags into your CSS and modify as is needed:

.password {
font-size : 12px;
border : 1px solid #cc9933;
width : 200px;
font-family : arial, sans-serif;
}
.pstrength-minchar {
font-size : 10px;
}

You can adjust a few more settings inside the jquery.pstrength-min.1.2.js script, such as messages, length/height of the bar, and so forth.

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 }