This document discusses database storage and retrieval. It describes two main methods of data access - sequential and direct. It also discusses distributed databases, storage media like hard disks and tapes, data encryption, database backups, data security measures, sorting and searching databases through queries. Queries use operators, fields and criteria to retrieve specific records. Structured Query Language (SQL) is also introduced as a common query language that uses keywords to search and retrieve data from databases.

1. Information Systems &
Databases
2.3) Storage and Retrieval

2. • A database management system
(DBMS) is the software used to
access, create and modify a
database.
• There are two main ways of
accessing data:
• sequential
• processes in sequence
• slower than direct
• cheaper
• e.g. magnetic tape

3. • direct
• can go straight to data
• faster than sequential
• more expensive
• A distributed database is a database
located at more than one site.
• It reduces data transmission costs.
• They need to be synchronised often.
• There are many types of storage
media. They can include:

4. • hard disc – a disc made of metal or
glass and covered in a magnetic
material (hard drives)
• optical disc – a plastic disc with a
reflective metal layer (CD, DVD,
Blu-ray)
• magnetic tape – a long strip of
plastic coated with a thin layer of
magnetic material; they are
inexpensive and sequential in
nature.

5. • Encryption is the process of
encoding data.
• Decryption is the process of
changing it back.
• Encrypted data is an effective way
to ensure data security.
• There are two main kinds of
encryption:
• asymmetric –require different
keys for encrypting and
decrypting

6. •
•
•
•
• symmetric –uses the same keys
for encrypting and decrypting
A DBMS contains backup and
recovery capabilities to guard
against data loss.
A backup is another copy of the
data that can be used to rebuild
the system.
Ideally, the backup should be in a
secure and different location.
Data security measures can also
include:

7. • user accounts and passwords
• personal ID objects like magnetic
cards
• biometric devices (e.g. fingerprint
scan, voice recognition)
• firewall – mix of hardware & software
• offline data access – no Internet
presence

8. • Sorting is the process or arranging the
data in meaningful way.
• Sorts can be either ascending or
descending.
• Ascending is:
• alphabetical
• smallest to largest
• earliest to latest
• Descending is the reverse.

9. • Searching the database is performed
through constructing a query.
• A query is a search for records that
meet certain conditions (or criteria)
• A simple query is constructed in the
form of :
<Field Name><Operator><Criteria>
• A common method is called query by
example (QBE)
• This is where you put characters into a
search field (with wildcards if needed)

10. • Wildcard characters represent one or
more unknown characters.
• An asterisk (*) substitutes for any
number of characters while a question
mark (?) substitutes for only one
character.
• Operators represent the action that is
performed in the query.
• They can be either relational or logical.
• Relational operators are symbols
indicating the relationship between
two expressions.

11. • They include: >, <, =, <>, <=, >=
• Logical operators are used to combine
queries.
• They include AND, OR & NOT.
• AND requires each condition to be met
while OR will be satisfied with any
condition.
• E.g. Year = 10 AND Gender = “M” will
return only Year 10 males while Year =
10 OR Gender = “M” will give all the
males in the school and all of Year 10

12. • A query language is a specialised
language designed to search a
database.
• Structured query language (SQL) is a
query language that uses the following
keywords:
• SELECT
• FROM
• WHERE
• ORDER BY
[COPY Table 2.4, p.59]

13. • When using SQL it is important to use
the correct syntax.
• This includes:
• keywords written in upper case
• fields separated by commas
• entity and attribute separated
by a full stop
• criteria or data items inside
quotation marks unless a
number

14. • E.g.
SELECT students.fname,
students.lname, classes.classname
FROM students, classes
WHERE Year>10 AND Gender = “F”
ORDER BY lname [asc], fname [asc]
[Complete LA 2 (c) and (d), p.63]
• A search engine is a database of
indexed websites.

15. • Indexing creates a table of information
about the location of data.
• This table is built by regularly scanning
the Internet for new sites and looking
at keywords and metadata.
• This scanning is often carried out by
programs called spiders, crawlers or
bots.