Enforcing minimal cardinality

• M-O
  • Children must have parents.
  • Primary key of parent is FK of child.
  • Require FK to be not-null
  • Implement cascading update/delete
• O-M
  • DBMS does not provide much help here.
  • Usually this is enforced through the use of triggers
  • A trigger is a code module which is invoked when an event (update, delete, create,...) occurs.
• M-M
  • Even more restrictive than O-M, so more work to enforce.
  • Try to avoid these if you can.
  • Chicken-and-Egg problems are common here:
    • I can't add a parent because I don't have a child, but I can't add a child because I don't have a parent, but I can't add a parent ...
  • Views come into play with these,
  • Read the paragraph on 230.
• You should probably read through the case study.