The Relational Model Flashcards
Describe an Entity
The important things in the real world that need to be modelled.
Give some examples of entities
People, places, objects, events
Describe an Attribute
Individual items of data associated with an entity.
Give some examples of an Attributes
Name, national insurance number, weight, date of manufacture
Describe a Relationship
Ways in which entities are connected.
Describe what is meant by constraints in regards to a relational database.
Rules which place limits on the data that is allowed
What is meant by ‘Domains’?
The set of possible values for a column, e.g.
- Date
- Varchar
- Integer
- Real
Define Entity Integrity
the ability to uniquely identify one row in a table.
Define a ‘Key’
an attribute (or group or attributes) that differentiates the rows.
What attributes count towards a good key?
Unique and doesn’t change,
Define a ‘Superkey’
Uniquely identifies a row.
Define a ‘Candidate Key’
Uniquely identifies a row and contains no redundant attributes
Define a ‘Primary Key’
Selected candidate key
Cardinality & Optionality = ____
Multiplicity
What does the JOIN operation do?
Returns a combination of columns from more than one table where some condition is true.
