replace the value in textbox in asp.net webform if the value not exists in sql server table

I have a textbox in asp.net web form with id name "txt_branch_name".

I want to change the value in the textbox to "" if the user enter value that not appear in sql server column in table.

how can i do it on the TextChanged event?

protected void txt_branch_name_TextChanged(object sender, EventArgs e) {

    }

Thanks a lot