Published on February 24, 2014
Information Systems & Databases 2.3) Storage and Retrieval
• 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
• 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:
• 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.
• 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
• • • • • 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:
• 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
• 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.
• 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)
• 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.
• 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
• 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]
• 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
• 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.
• 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.
Notes on 2.3 of the HSC IPT course ... 1. Information Systems & Databases 2.3) Storage and Retrieval
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