Week 3 Flashcards
What does UNION do?
What does DIFFERENCE do?
What does entity integrity mean?
What does relational integrity mean?
What are superkeys?
An attribute or set of attributes which identify a particular tuple.
Their only property is that they are unique.
What are candidate keys?
These are a subset of superkeys that have no subset that can be a superkey.
They are also referred to as MINIMAL superkeys.
A primary key is chosen from the candidate keys.
What are the attributes needed to allow a union between relations?
What are secondary keys?
What does SELECT/RESTRICT do? Notation
Returns a horizontal subset within a relation.
Sigma
What does PROJECT do? Notation?
Returns a vertical subset in a relation.
Pi
Are candidate keys superkeys? Whats the difference?
Yes. Candidate keys have no redundant components. All the components are required to indentify whatever the candidate key is for.
What does a relation consist of?
- Heading
- Body
What is a relation heading/relational schema?
Consists of all fixed set of attributes for relation R
What are tuples?
Instances of the heading,
What is a relation body/instance
Consists of a time varying set of n-tuples.
What is referred by relation cardinality?
The number of tuples
What is the relation degree?
The number of valies in tuple (number of attribute)
What are the fundamental relation properties?
- NO duplicate tuples
- Tuples are UNORDERED within a relation
- NO ordering of attributes within a tuple
What are the fundamental relation properties?
- NO duplicate tuples
- Tuples are UNORDERED within a relation
- can only be accesed via data within tuple
- NO ordering of attributes within a tuple
- Tuple values are atomic (cant be divided)
Are tables and relations the same?
NO, tables are visual/tabula representations of relations. THEY ARE NOT THE SAME.
Are multivalued/repeating attributes allowed in a tuple?
No
What is functional dependancy?
When one attribute determines another attribute.
Attribute A can be be used to determine attribute B.
Attribute A = unique
What are alternate keys?
These are candidate keys that were NOT chosen as PKs
What are alternate keys?
These are candidate keys that were NOT chosen as PKs