Week 3

Question 1

What does UNION do?

Question 2

What does DIFFERENCE do?

Question 3

What does entity integrity mean?

Question 4

What does relational integrity mean?

Question 5

What are superkeys?

Answer 5

An attribute or set of attributes which identify a particular tuple.

Their only property is that they are unique.

Question 6

What are candidate keys?

Answer 6

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.

Question 7

What are the attributes needed to allow a union between relations?

Question 8

What are secondary keys?

Question 9

What does SELECT/RESTRICT do? Notation

Answer 9

Returns a horizontal subset within a relation.

Sigma

Question 10

What does PROJECT do? Notation?

Answer 10

Returns a vertical subset in a relation.

Pi

Question 11

Are candidate keys superkeys? Whats the difference?

Answer 11

Yes. Candidate keys have no redundant components. All the components are required to indentify whatever the candidate key is for.

Question 12

What does a relation consist of?

Answer 12

• Heading

- Body

Question 13

What is a relation heading/relational schema?

Answer 13

Consists of all fixed set of attributes for relation R

Question 14

What are tuples?

Answer 14

Instances of the heading,

Question 15

What is a relation body/instance

Answer 15

Consists of a time varying set of n-tuples.

Question 16

What is referred by relation cardinality?

Answer 16

The number of tuples

Question 17

What is the relation degree?

Answer 17

The number of valies in tuple (number of attribute)

Question 18

What are the fundamental relation properties?

Answer 18

• NO duplicate tuples
• Tuples are UNORDERED within a relation
• NO ordering of attributes within a tuple

Question 19

What are the fundamental relation properties?

Answer 19

• 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)

Question 20

Are tables and relations the same?

Answer 20

NO, tables are visual/tabula representations of relations. THEY ARE NOT THE SAME.

Question 21

Are multivalued/repeating attributes allowed in a tuple?

Answer 21

No

Question 22

What is functional dependancy?

Answer 22

When one attribute determines another attribute.

Attribute A can be be used to determine attribute B.

Attribute A = unique

Question 23

What are alternate keys?

Answer 23

These are candidate keys that were NOT chosen as PKs

Question 23

What are alternate keys?

Answer 23

These are candidate keys that were NOT chosen as PKs

Question 24

What is a surrogate key?

Answer 24

This is an attribute that is soley present to act as a primary key.

Question 25

Should primary keys be free of semantics?

Answer 25

Yes, they should not provide any additional information (especially important info) E.g. Although TFN could be a PK, best not to use TFN in a public setting as it is private info.

Question 26

What is a NULL?

Answer 26

This refers to when there is NO VALUE. NOTE: 0 IS NOT A NULL VALUE

Question 27

Do NULLs exist in the relational model?

Answer 27

NO, they were only implemented by the relation database system.

Question 28

What is the naming convention for relations?

Answer 28

- Name is all caps and singular - PK is start of tuple and underlines - Rest of attributes NOT ordered

Question 29

Is there redundant data in a relational database?

Answer 29

YES, a minimal amount in foreign keys/composite keys.

Question 30

What are the functions of a PK?

Answer 30

- To ensure that a tuple is unique | - To act as a foreign key to relate (logical connection between) tuple

Question 31

What are the rules for FKs?

Answer 31

- They MUST point to a COMPLETE PK | - If no match with full PK then NULL

Question 31

What are the rules for FKs?

Answer 31

- They MUST point to a COMPLETE PK | - If no match with full PK then NULL

Question 32

Do FKs need the same name as PKs?

Answer 32

No, they can have different names.

Question 33

What is domain/column integrity?

Answer 33

All values in a given column must come from the same domain (same datatype and within the range)

Question 34

What does a "closure" property refer to?

Answer 34

Queries on relations produce relations.

Question 34

What does a "closure" property refer to?

Answer 34

Queries on relations produce relations.

Question 35

What is procedural?

Answer 35

You have to specify the instructions as well as the order of instructions.

Question 36

What are the basic operations in relational algebra?

Answer 36

single relation: selection, projection two relations: - cartesian product, join - union - intersection - difference - division

Question 37

What does JOIN do?

Answer 37

Combines data from two or more relations based on a common attribute or attributes

Question 38

Types of joins?

Answer 38

- Theta (generalised join) - Equi - Natural

Question 39

What is a theta join?

Answer 39

RELATION.attribute F RELATION2.attribute F is an operator ( etc.)

Question 40

What is a natural join?

Answer 40

Projection of the results from an equijoin ???

Question 41

What is an equijoin?

Answer 41

A join where two attributes are made to be equal

Question 42

What is a canonical query?

Question 43

What is a canonical query?

Question 44

Ways to be more efficient when querying?

Answer 44

Grabbing data as early as possbile