What's the difference between identifying and non-identifying relationships?

Question

I haven\'t been able to fully grasp the differences. Can you describe both concepts and use real world examples?

Answer 1

Here's a good description:

Relationships between two entities may be classified as being either "identifying" or "non-identifying". Identifying relationships exist when the primary key of the parent entity is included in the primary key of the child entity. On the other hand, a non-identifying relationship exists when the primary key of the parent entity is included in the child entity but not as part of the child entity's primary key. In addition, non-identifying relationships may be further classified as being either "mandatory" or "non-mandatory". A mandatory non-identifying relationship exists when the value in the child table cannot be null. On the other hand, a non-mandatory non-identifying relationship exists when the value in the child table can be null.

http://www.sqlteam.com/article/database-design-and-modeling-fundamentals

Here's a simple example of an identifying relationship:

Parent
------
ID (PK)
Name

Child
-----
ID (PK)
ParentID (PK, FK to Parent.ID) -- notice PK
Name

Here's a corresponding non-identifying relationship:

Parent
------
ID (PK)
Name

Child
-----
ID (PK)
ParentID (FK to Parent.ID) -- notice no PK
Name