Chapter 2 Flashcards
(37 cards)
What data represents?
represents a fact or character-istic about an object of interest to us
File
A collection of facts that describes a particular object
Record
a row of a file; this represents the data that we have on one instance of the file object
Field
a column of a file; this represents the data we have for one characteristic, or attribute, of all instances of the file object
A key field for a data table contains?
data values that are unique in the file; that is, each record will always have a different value for that field than all other records
fundamental operations -Read
looking at a record’s contents without changing them
fundamental operations-Insert
adding a new record to the file, as when a new salesperson is hired
fundamental operations-Update
changing one or more of a record’s field values
fundamental operations-Delete
deleting a record from the file, as when a salesperson leaves the company
Primary Memory
where computers execute programs and process data
- Also known as RAM (random access memory)
- Very fast data access
- Has several drawbacks
- relatively expensive
- not transportable
- volatile (contents are lost without power)
Secondary Memory
can store vast volumes of data and the programs that process them
Access Arm Mechanism
The basic disk drive has one access arm mechanism with arms that can reach in between the disks
At the end of each arm are two read/write heads
The platters spin, all together as a single unit, on the central spindle, at a high velocity
Steps in Finding and Transferring Data- Seek Time
The time it takes to move the access arm mechanism to the correct cylinder from the cylinder at which it’s currently positioned
Steps in Finding and Transferring Data- Head Switching
- Selecting the read/write head to access the platter for the required track of the cylinder
Steps in Finding and Transferring Data- Rotational Delay
- Waiting for the desired data on the track to arrive under the read/write head as the disk is spinning
Steps in Finding and Transferring Data- Transfer Time
The time to actually move the data from the disk to primary memory once the previous 3 steps have been completed
Data Retrieval Methods– Sequential Access
- the retrieval of all or most of the records of a file one after another, in some sequence, starting from the beginning, until all of the required records have been retrieved
However, this method is very inefficient when one or a few records are sought in no set order
Data Retrieval Methods- Direct Access
the retrieval of a single record of a file, or a subset of the records of a file, based on one or more values of a field or a combination of fields in the file
Sometimes called random access
Requires software that will take advantage of the hardware’s capabilities and store and retrieve the data in such a way that it accomplishes direct access
Indexing
Principal is the same as that governing the index in the back of a book
An index file, separate from the main data file, is created
Hashing
is a file access method that allows for direct access of data without creating indexes
Hashing–division-remainder
Divide the key value of the desired record by the number of record locations that have been reserved; discard the quotient, and use the remainder to specify where to locate the record
Database Characteristics
- Contain ordered collections of data
- Contain related or linked data elements
- Designed to satisfy specific information needs
- Provide shared data access to multiple users
Data repository
storage unit where physical data files are kept
Data dictionary
location where definitions of data (metadata) are maintained
