Writing for Wednesday, October 23

• The Relational Model:
  • Used by all major commercial database systems
  • Very simple model
  • Query with high-level languages--simple yet expressive
  • Efficient implementations

• Database = set of named relations (or tables)
  • Each relation has a predefined set of named attributes (or columns)
  • Each tuple (or row) has a value for each attribute.
  • Each attribute has a type (or domain)
    • i.e. ID is an int, Name is a string, GPA is a float, etc.

• Schema-- structural description of relations in database.
  • Set up in advance
  • The framework/empty table(s)

• Instance-- actual contents at given point in time
  • Change over time

• Null-- special value for "unknown" or "undefined"
  • Queries might exclude tuples that contain null values
    • E.g. GPA > 3.5 OR GPA <= 3.5 would exclude Craig

• Key-- attribute whose value is unique in each tuple e.g. ID in the Student table
  • Or set of attributes whose combined values are unique e.g. Name and State in the College Table

Student       

College

• Creating relations (tables) in SQL:

      > Create Table Student(ID, name, GPA, photo)

      > Create Table College

                   (name string, state char(2), enrollment integer)

• Relational Model Summary:
  • Used by all major commercial database systems
  • Very simple model
  • Query with high-level languages: simple yet expressive
  • Efficient implementations