invoice.InvoiceNo like'ajay%' or Customer.MobileNo like'ajay%'
how can you check Ajay with InvoiceNo, Mobile No ?
you should know Ajay is Customer name not Mobile No, invoice No.
change you way of accepting input from user. Use dropdownlist containing search fields CustomerName, InvoiceNo, Mobile No.
first select search criteria and then enter desired value in textbox.