week 3 : relational data model

Question 1

what is a relational data model?

Answer 1

• based on the mathematical concept of a relation
• organizes and represents data in a form of a table

Question 2

relation

Answer 2

a table with rows and columns

Question 3

tuple

Answer 3

row of a relation

Question 4

attribute

Answer 4

a named column of a relation

Question 5

degree (of a relation)

Answer 5

number of attributes in a relation

Question 6

cardinality (of a relation)

Answer 6

number of tuples in a relation

Question 7

domain

Answer 7

set of allowable values for each attribute

Question 8

relational model terminology

Answer 8

• base relation
• view

Question 9

base relation

Answer 9

a named relation corresponding to an entity in conceptual schema, whose tuples are physically stored in a database

Question 10

view

Answer 10

• dynamic result of one or more relational operations operating on base relations to produce another relation
• a virtual relation that does not necessarily actually exist in the database but is produced upon request (at time of request)
• dynamic : changes made to base relations that affect view attributes are immediately reflected in the view
• contents of a view : query on one or more base relations

Question 11

properties of relations (1-3)

Answer 11

1) relation name is distinct from all other relation names in relational schema

2) each cell of relation contains exactly one atomic (single) value

3) values of an attribute are all from the same domain

Question 12

properties of relations

Answer 12

4) each attribute has a distinct name

5) each tuple is distinct (no duplicates)

6) order of attributes has no significance

7) order of tuples as no significance

Question 13

relational keys

Answer 13

• candidate key
• primary key
• alternate key
• foreign key

Question 14

candidate key

Answer 14

• any minimal set of attributes that can uniquely identify each tuple in relation
• relation may have 1 or more candidate keys
• consists of a single attribute or multiple attributes
• multiple attribute keys = composite keys

Question 15

primary key

Answer 15

one candidate key –> primary key whose values are used to identify tuples uniquely in a relation

Question 16

alternate key

Answer 16

candidate keys that are not selected to be the primary key

Question 17

primary key criteria

Answer 17

• minimal set of attributes
• less likely to have its values changed
• less likely to lose uniqueness in the future
• fewest characters
• easier to use from the user’s POV

Question 18

foreign key

Answer 18

• attribute which provides a logical link between tables
• an attribute/set of attributes within one relation (r1) that matches the candidate key of some relation (r2), (can be the same key)