Alternate scenarios in Use Case Diagram (If conditions)

Question

Do we include if conditions/alternate scenarios in a use case diagram?

Actually, I am making a use case diagram for a book order application. It has this condition that if the number of copies ordered are less than the copies of book available then it will create a shipping as well as back order and if all copies are available then it will only create a shipping order. So will this condition be included with "extend" in Check copies of book in USE CASE DIAGRAM? Or will this condition be specified only in a sequence diagram.

Answer 1

Use cases are about added value the system under consideration delivers to its primary actors. A use case contains activities which describe various paths how this goal can be achieved. But that's not exposed on a use case diagram. «includes» is to re-use a large goal within another use case unconditionally. And «extends» is used for optional parts which themselves must be "sub-goals". To actually show the conditional paths you create one or more activity diagrams per use case.

In your case the "Create shipping order" as single use case will be an extension of "Order book". When and under which conditions this extension is performed is hidden in the activities of "Order book". Your use case diagram only shows the «extends» relation between both UCs.