Creating one-to-many maps
When an entity can be associated with one or more entities of another type, and each of these entities is associated with at most one entity of the first type, we call that one-to-many. It is one of the more basic kinds of relation, and, if looked at from the other endpoint, it becomes a many-to-one relation. Some examples of this include the following:
- A blog and its posts
- A parent and their children
- A folder and its sub-folders
- An order and its details (items included)
You may notice that there is one difference: in some of these relations, the many side cannot exist without the one—for example, a child without a parent–while in others, it can—there can be a folder without a parent folder.
This is easy to represent in the ...