APEX

I need to create an alert based on if a fields data exists in database when the user enters text in the text field. Has anyone ever created an alert message if the value entered exists in database already. I only see Javascript examples to prompt the user what data was entered or to disable a property based on a value = '80' or any value. But I need every value checked.

<code>
var lCheck = html_SelectedValue('P2_DEPARTMENT_ID')!='80;
<code>

I can check any specific value if its harded code but, I need to check every value if the user types in the text field then tabs off the field and if that value exists in the database then the alert message pops up and lets the user know this value entered exists in database. I can not hard code hundreds of values based on the example above. I hope someone can help me with this matter, thanks ahead of time.