Database Questions Pg.80
Database Questions Pg.80Alex Swan
Homework Review Questions
3.1 Define the following terms as they apply to the relational model of data:
Domain – Set of atomic (indivisible) values.
Attribute – it describes a component of the database, such as a table or a field.
N-tuple – in mathematics, an ordered set of n elements called its components or coordinates.
Relation Schema – It is representation of database highlighting relationships that we have created.
Relation State – set of tuples that have the same attributes.
Degree of a relation – number of attributes n of its relation schema.
Relational Database Schema – Tables, columns and relationships that make up a relational database.
Relational database state – is a union of all the individual relation states, whenever the database is changed a new state arise.
3.2 Why are tuples in a relation not ordered?
- A Relation is defined as a set of tuples. Mathematically, elements of a set have no order among them: tuples in a relation do not have any particular order. In other words, a relation is not sensitive to the ordering of tuples. Tuple ordering is not part of a relation definition because a relation attempts to represent facts at a logical or abstract level.
3.3 Why are duplicate tuples not allowed in a relation?
- Duplicate tuples are not allowed in a relation because it violates the specifications of the relational integrity constraints, particularly the key constraint which states that no two tuples can have the same values for their attributes at any relation state of a database.
3.4 What is the difference between a key and a superkey?
- Superkey is any subset of attributes that uniquely identifies the tuples of a relation. This subset need not be minimal, that is, one can remove some attributes from it and it is still uniquely identifying. If all redundant attributes are removed you get a subset what is called primary key. A key (better: primary key) is the minimal subset of...