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What is a static constraint and a transition constraint.

  • A Static constraint checks that an incoming transaction will not change a database into an invalid state.
  • Example: In a database of students and courses (the textbook example) the static constraint” students can only take currently offered modules”, would prevent the user from entering data that included a new module.
  • A Transition constraint is a rule that relates to given states of a database. A transition can be denoted by a pair of states.
  • Example: In a database of students and courses, a transition constraint could be “the number of modules taken by a student must not drop to zero for a given term”. Thus, if the data relating to a specific student and module is to be deleted, the transition constraint would first check that this would not cause an invalid transition (the number of modules being zero).