How can I delete photos from camera roll and leave in an album?
You can't. iOS uses a simple photo database, very similar to iPhoto. Primary storage for all photo image data and metadata (time, place, exposure, etc) is the camera roll. This system lets you create the illusion of multiple albums, and storing photos in the albums.

But from a database perspective, your album is just a table with the image ID of every photo you place in it, creating a link back to the image data in the camera roll.

You can delete photos from albums, thereby removing them from the table. 

When you delete a photo from the camera roll, you delete the photo from the system.