That reason, in fact, is why sub-classes are needed in data modeling. Attribute and relationship inheritance occurs through the construction of sub-classes of objects from existing classes. We can then customize those lower-level objects with their own particular attributes and relationships. Discuss why concurrency control is needed, and give informal examples. This situation leads directly to concurrency control issues, since it is possible that one person s transaction against a particular database element may not complete prior to the start of another users transaction against that very same database element.
Numerous examples exist in all applications that are based on multi-user database systems.