Relational Model

Attributes

• Domain: Set of allowed values for each attribute
• Attribute values are normally atomic
• Attributes can be null

Relation Schema

• , , … , are attributes

• = , , … , is a relation schema

  • instructor = (ID, name, dept_name, salary)

• The current values of a relation are specified by a table.

• Each row in a table is a tuple

• The order of tuples (order of rows) doesn’t matter

Database

• Database consists of multiple relations
• Good Design:
  • Avoid repetition of information
  • Avoid null values

Keys

• Keys can be used to uniquely identify a tuple (A row in a table)
• K R
• A key consists of a subset of elements from a relation schema. (A key can have one or more column)
• Super Key: values for K are sufficient to identify all unique tuple of each possible relation
  • Ex: {ID}, {ID,name}
• Candidate Key: a minimal super key
  • Ex: {ID}
• Primary Key: Any candidate key can be used as a primary key, But it’s better to use a meaningful, stable, small key as a primary key
  • It’s better to use {ID} as a primary key than {Name} because names are not stable
• Foreign Key: A value in one relation appearing in another relation
  • Referencing Relation
  • Referenced Relation