COMP2004 - DB Conceptual modelling

Question 0

Entity Relationship Diagram

Answer 0

Specify entities in a system along with their relationships

Question 1

Conceptual modelling

Answer 1

• model objects
• identify;
  Relationships btwn them
  Their cardinality and participation
  Identify strong and weak entities

Question 2

Modelling

Answer 2

• objects - from nouns in UML
• attributes - properties of objects
• associations - relationships btwn objects
• cardinality - 1 to many, many to many
• participation - optional, mandatory

Question 3

Weak entities

Answer 3

Can only exist in the presence of a strong entity ( composite notation)
so on UPDATE/DELETE CASCADE

Question 4

Composite notation

Answer 4

It is only called composite if the objects it refers to are really its parts, i.e. have no independent existence. For details, see the aggregation section below.

Question 5

High level model - Conceptual model

Answer 5

Understanding at a high level the different concepts in the system

Question 6

Middle level model - Logical model

Answer 6

Adding details to the data - middle step between the two

Question 7

Low level model - physical model

Answer 7

how it is implemented in the database

Question 8

Functional dependency

Answer 8

Constraint btwn two sets of attributes in a relation from a database

Question 9

Relation

Answer 9

Is a set of tuples where each element is a member of a data domain. Each distinct domain used in the definition of a relation is called an attribute

Question 10

Relation

Answer 10

Attribute
| | | | |
—————————–
Tuple | | | | |
—————————–
| | | | |
—————————–
The whole thing is relation

Question 11

Deriving tables

Answer 11

Objects become tables with the primary key chosen appropriately

Question 12

Tuple/ Row

Answer 12

A sequence or ordered list of finite length

Question 13

Column/Attribute

Answer 13

Defines properties of an individual

Question 14

Relation

Answer 14

Table

Question 15

Tuple

Answer 15

Row

Question 16

Attribute

Answer 16

Column

Question 17

Domain/Type

Answer 17

Attribute type

Defines how a given item is constrained

Question 18

Schema

Answer 18

Defines a structure of a database
Name ({ as })
Degree of relationship is the number of attributes in a table
Known as relation intension

Question 19

Model

Answer 19

Structure
Operations
Constraints
Eg relational, semistructure (xml)

Question 20

Relational model

Answer 20

Model
Schema
State

Question 21

Super keys

Answer 21

Set of attributes which uniquely identify all other attributes
Values for the attributes in a superkey will identify one unique record

Question 22

Candidate key

Answer 22

A subset of the super keys which contain the minimal number of attributes to uniquely identify a record

Question 23

Primary key

Answer 23

One of the candidate keys which the user has specified will be the unique identifier for a record. Irreducible. Not null
PRIMARY KEY FIELDS ARE IN CAPITAL LETTERS

Question 24

Foreign key

Answer 24

Refers to a candidate key in another table

Question 25

Functional dependency

Answer 25

Where a subset of attributes A2 is determined by another subset of attributes A1 such that A2 depends on A1. A1(determinant) ->A2 (dependent) Means A1 determines A2 or A2 is determined by A1

Question 26

ACID

Answer 26

Atomicity Consistency Isolation Durability

Question 27

Atomicity

Answer 27

All the operations in the transaction occur or nothing occurs An operation must start and then run to dinish without being interrupted or stopped along the way. ATOMOS=indivisible

Question 28

Consistency

Answer 28

Any transaction will bring the state of the database from one valid state to another (neither state can invalidate any of the defined rules). Eg. Transaction transferring funds ensures that sum of both accounts is the same before and after.

Question 29

Isolation

Answer 29

Events within a transaction are hidden from other transaction running concurrently. Uses synchronisation techniques. Concurrent operations on the database must leave the system in a state as if they were run sequentially.

Question 30

Durability

Answer 30

Once a transaction has been completed and committed it must guarantee these results survive subsequent malfunctions. The user must have a guarantee that what has happened has actually happened. A change committed to the database must be permanent, regardless of other operations, power failure, crashes etc

Question 31

Structure of a transaction

Answer 31

START A TRANSACTION SQL STATEMENTS COMMIT = the statements are installed permanently or ROLLBACK - the transaction aborts, database is restored to the original state

Question 32

Two phase locking

Answer 32

Is the ensuring that within each transaction all lock actions precede all unlock actions. This ensures that any legal schedule of transactions remains conflict serialisible.

Question 33

Semi structured data

Answer 33

Form of structured data that does not have a matching schema. It still has correct tags (is still well fromed) but tags might not be consistent across whole document.

Question 34

Well formed XML

Answer 34

Is syntactically correct XML | Conforms to XML spec and can be parsed

Question 35

Valid XML

Answer 35

Is gramatically correct for a given grammar. | Valid XML must also be Well Formed XML

Question 36

DTD

Answer 36

Document Type Definition