Intro

• Chapter six assumes that you have an ER design.
• To turn this into a database you mus
  • Replace entities and attributes with tables an columns
  • Represent relationships and maximal cardinalities with foreign keys
  • Represent minimum cardinality by defining actions to constrain activities on values of primary and foreign keys.
• Create a table for each entity
• Select a primary key.
  • This facilitates searching.
  • Used in creating indexes as well
  • Ideal:
    • Short
    • Numeric
    • Fixed
  • If none exist, use a surrogate key.
  • Disadvantages to surrogate keys
    • The numbers are meaningless to the institution.
    • They may eventually become meaningful, but at least initially they are meaningless.
    • They are a problem when databases are merged or information is shared.
    • This is a problem of duplicate, misaligned, missing or different keys for the same data in different databases.
• Candidate Keys
  • Sometimes known as alternate keys.
  • Are used in the normalization process.
• Specify column properties
  • Null or not
    • NULL means no data has been supplied.
    • Primary keys may never be NULL
    • Alternate keys may be, see discussion
  • Data type
    • Matches available data in DBMS
  • Default Value
    • General variable initialization problem.
  • Data Constraints
    • Normal bounds checking.
    • Logic implementing policies and such.
    • Referential Integrity Checks.
• Verify normalization
• Check things out as carefully as possible with the client.
• Write it all down so the team, and future maintainers know what was done and why.