Hi,
welcome to Hyperic HQ.
You do not need to develop a plugin.
At first create a New Server (http://support.hyperic.com/display/DOC/ui-Inventory.Server.New). As service type please choose SQL Query. Set up the configuration properties for your database server and query.
The second step is to define a proper alert (http://support.hyperic.com/display/DOC/ui-Alert.New).
hth,
Mirko