Section 4 - Exchanging Data

Question 1

Chapter 16 - Database concepts

What is a;
A) Flat file database
B) Relational database

Answer 1

A) A flat file database consists of asingle file. It might be suitable to hold the names and addresses of all memebersof a sports club, or information about all the dvds in your personal collection

B) In a relational database, a separate table is created for each entity indentified in the system. Foreign keys are used to link the 2 tables

Question 2

Chapter 16 - Database concepts

Define the following keys:
A) Primary Key
B) Secondary Key
C) Foreign Key
D) Composite Key

Answer 2

A) A primary key exists to uniquely identify an entity, as such, that key needs to be wholey unique as to not have duplicates.

B) An index of all the primary keys in the database, and there the record is held, is automatically maintained by the database software.
- When it comes to searching/sorting/filtering a database, a secondary key is likely to be used

C) A foreign key is an attribute that creates a join between 2 tables. It is the attribute that is common to both tables and the primary key in on table is the foreign key in the able to which it is linked.

D) This is a primary key which consists of more than one attribute.

Question 3

Chapter 16 - Database concepts

Explain what is ment by Referential integrity

Answer 3

When tables are linked in a relational database, it is important to ensure that, a particular component is not deleted if it is used in a product in the product table. This is known as referential integrity.

Question 4

Chapter 17 - Relational databases and normalisation

Explain what Indexing is

Answer 4

• In order that a record with a particular primary key can be located quickly in a database, an index of primary keys will be automically mainted by the DB software, giving the position of each record accodring to its primary key.
• One or more secondary indexes ay be defined when the database is created, for any attribute that is often used as a search criterion.

Question 5

Chapter 17 - Relational databases and normalisation

Explain what Normalisation is and why it is used.

Answer 5

What it is: Normalisation is a process used to come up with the best possible design for a relational database:

Why its used:
- No data is unnecessarily duplicated
- Data is consistent throughout the database
- The structure of each table is flexible enough to allow you to tenter as many or as few items as required
- The structure should enable a user to make all kinds of complex queries relating data from different tables
- Faster searching and sorting

Question 6

Chapter 17 - Relational databases and normalisation

Explain what it means for a table to be in;
A) First normal form
B) Second normal form
C) Third normal form

Answer 6

A) A table is in first normal form if it contains no repeating attributes or groups of attributes ( attributes must be atomic )

B) A table is in second normal form if it is in first normal form and contains no partial dependencies.
- A partial dependency would mean that one or more of the attributes depends on only part of the primary key, which can only occur if the primary key is a composite key

C) A table is in third normal form if it is in second normal form and contains no ‘non-key dependencies’.
- A non-key dependency is one where the value of an attribute is determined by the value of another attribute which is not part of the key

Question 7

Chapter 20 - Transaction processing

Describe what ACID is

Answer 7

ACID is a set of properties that guarantees that transactions are processed reliably.

• Atomicity: Atomicity requires that a transaction must be processed in its entirety or not at all
• Consistency: The consistency property ensures that no transaction can violate any of the defined alidation rules for maintaing the integrity of the database
• Isolation: The isolation property ensures that concurrent execution of transactions lead to the same result as if the transaction were processed one after the other.
• Durability: The durability property ensures that once a transaction has been committed, it will remain so, even in the even of a power cut.

Question 8

Chapter 20 - Transaction processing

Describe what record locking is and the issues that it brings

Answer 8

What it is: Record locking is the techniqe of preventing simultaneous access to objects in a database in order to prevent updates eing lost or inconsistencies in the data arising.

Problems: If 2 users are attempting to update 2 records, a situation can arise in which niether can proceed, known as a deadlock. To combat this, Serialisation, timestamp ordering or commitment ordering may b used

Question 9

Chapter 20 - Transaction processing

Describe what Serialisation is

Answer 9

Serialisation: This is a technique which ensures that transactions do not overlap in time and therefore cannot intefere which each other or lead to updates being lost. A transaction cannot start until the previous one has finished. This can be implemented using timestamp ordering

Question 10

Chapter 20 - Transaction processing

What is:
A) Timestamp Ordering
B) Commitment Ordering

Answer 10

A) Timestamp Ordering: Whenever a transaction starts, it’s given a timestamp, so that if 2 transaction affect the same object, the transaction the earlier timestamp should be applied first.
- Every object in the database will have a read timestamp and a write timestamp.
- If the read timestamp has changed from when the transaction started when being written to, it will know another transaction is taking place

B) Commitment Ordering: Transactions are ordered in terms of their dependencies on each other as well as the time they were intiated. It can be used to prevent deadlock by blocking one request until another is completed

Question 11

Chapter 20 - Transaction processing

Describes some methods of;
A) Capturing Data
B) Selecting/Managing data
C) Exchanging data

Answer 11

A) Methods include;
- Manual entry
- Magnetic ink character recognition (MICR, used for cheques at banks)
- Optical mark recognition (OMR, and others types of forrm using OCR)
- Smart card readers
- Scanners
- Barcode readers

B) Data may be selected before it is even added to a database, depending on whether or not is matches specifcied criteria.
- Once in the database, SQL may be used to select data from different tables

C) A common method of transferring data between computer systems without the need for human intervention is EDI ( Electronic Data Interchange ) Using standardised message formatting, document can be exchange electronically.
- Transaction software processes the information and software on the receiving end looks up details of, for example, items to be purchased, price, buyer’s name etc.