Mod 5 - ERD Conversion

Relation:

• A named, two-dimensional table of data

Requirements for a table to qualify as a relation:

• It must have a unique name
• Every attribute value must be atomic (not multivalued)
• Every row must be unique (no duplicate rows allowed)
• Attributes (columns) must have unique names
• The order of the columns and rows must be irrelevant

Foreign Keys vs Primary Keys

• Keys are special fields that serve two main purposes:
  • Primary keys are unique identifiers of the relation in question. Examples include employee numbers, SSN, etc.
  • Foreign keys are identifiers that enable a dependent relation (on the many side of a relationship) to refer to its parent relation (on the one side of the relationship)
• Keys can be simple (a single field) or composite (more than one field)
• Keys usually are used as indexes to speed up the response to user queries

Relational Schemas