Database Questions Pg.80

956 Words Oct 16th, 2012 4 Pages
Alex Swan
Mr. Davis
9/18/12
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
…show more content…
Therefore, the super superkey is always the set of all attributes of a relation.
3.5 Why do we designate one of the candidate keys of a relation to be the primary key? - When there are more than one keys in a relation schema of database, all these keys are referred to as candidate keys. But, a particular candidate key is called as a primary key over the other candidate keys. This primary key is generally a single attribute or a smaller number of attributes. We designate so because it becomes fairly easier to deal with a database when we can have a single distinct key for a particular relation instead of having more than one key.
3.6 Discuss the characteristics of relations that make them different from ordinary tables and files? - Ordering of tuples in a Relation: The tuples are not considered to be ordered, even though they appear to be in the tabular form. - Ordering of attributes in a relation schema R and of values within each tuples: We consider the attributes in R(A1, A2, .., An) and the values in t=<v1, v2, .., vn> to be ordered. - Values in a tuple: All values are considered to be atomic or indivisible. A special null value is used to represent values that are unknown or inapplicable to certain tuples.
3.7 Discuss the various reasons that lead to the occurrence of NULL values in
Open Document