In some cases, you need to get quick information from a database, but you don’t want to process an entire page. For example, you may have a form for requesting information about an order. The form might have multiple fields, one of which is the order number. The order number is not required, but if it’s filled in it must match an existing order number in the database. Rather than waiting to check the order number until the user fills out and submits the entire form, you can use Ajax to flag a bad order number as soon as the user tabs out of that field.