How to use summary field conditionally for display? I have created a content type Band with a Description field (as Long Text and Summary). When I am viewing the Band page, I want to see the full description field. And when I am viewing a listing of bands, I want to see a summary description or the first hundred characters if the summary description is missing.

How do I show appropriate Description for the Band page and the listing pages?
The Description text field needs to be of type Long text and summary.

With view modes (admin/structure/types/manage/band/display) you can configure how the fields must be rendered.

In the view mode Teaser you could set the format to Summary or trimmed and in other view modes to Default.

When viewing the content type Band as a full page you can render the view mode Default. When rendering a list of Bands with views you can set the view mode to Teaser.