By default you can press Control + f (or otherwise see set cedit) when on the Vim command-line, which opens the command-line window where you can edit the command using normal-mode Vim editing keys.
Enter will run the command or Control + c will return you to the standard command-line.
So in your specific example, you could press Control + f on the Vim command-line then db and it would do what you want.
When I have to do more sophisticated editing commands I use the above approach because I’m more familiar with Vim editing keys than I am with the alternatives. I also find the normal-mode vim keys more powerful.
See :help c_ctrl-f for more information.