2.1 Data Input and Sources of Error Data Input Can be done through: Keyboards Sensors Scanners Sound...

2.1 Data Input and Sources of Error

Data Input Can be done through:

Keyboards Sensors Scanners Sound recorders Digital video cameras


Sources of Error Mistakes may be made during data entry. Three different sources of errors caused by manual

input:Error Source of error Example

Data source error Data source providers provide incorrect data.

An interviewee reports an incorrect telephone number.

Transcription error

Data is read or typed in incorrectly. ‘ 1’ as ‘l’ or ‘o’ as ‘0’

Transposition error

Two consecutive digits are swapped.

Type 61 when you intend to type 16.

Sources of error caused by manual input


Garbage-in-garbage-out (GIGO) Erroneous data always produces inaccurate information. Waste human resources while producing useless

information. Steps must be taken to identify inaccurate data during

the process of data input.


Garbage-in-garbage-out (GIGO)

Garbage in

Transcription error

An example of garbage-in-garbage-out

The position numbers in all exam reports are wrong. They are regarded as errors and they are garbage.

Garbage out


Garbage-in-garbage-out (GIGO)

Garbage in

Transcription error

The position numbers in all exam reports are wrong. They are regarded as errors and they are garbage.

An example of garbage-in-garbage-out

Garbage out

2.2 Data Control

Two types of data control to reduce data errors:

Data

validationData

validationData

verification

Data verification

Data Validation The process of comparing data with a set of rules or

values to make sure that the data is reasonable and valid

Can detect some invalid data in the source documents

2.2 Data Control

Data Validation

Different types of validity checks and their uses

Validity check Function Example

Field presence check

Ensure that all necessary fields are present.

The student number must be included in the student record.

Field length check

Ensure that the data has the correct number of characters or digits.

A domestic telephone number in Hong Kong must have 8 digits.

Range check Ensure that the data value is within a pre-determined range.

The mark of a test should range from 0 to 100.

Fixed value check

Ensure that the data conforms tobe one of the values in a predefined list.

The gender of a student must be either ‘Male’ or ‘Female’.

Format check Ensure that the form of data follows some known patterns.

The e-mail address must be a combination of a user name and a domain name with the symbol ‘@’ in between.

2.2 Data Control

Data ValidationValidity

check Function Example

Type check Ensure that the data types are correct.

The students’ examination marks should be numbers but not characters, while their names should be characters but not numbers.

Check digit

It is used for self-checking. With numeric data, a check digit is calculated using a mathematical formula and this is then attached to the end of the number. When the check digit of the number matches the digit calculated from the number, the number is confirmed valid.Parity check is a common application of check digit in digital data transmission.

The check digit of ISBN.

Different types of validity checks and their uses

2.2 Data Control

Data Verification A control used to check whether the inputted data

matches that in the source document Two methods commonly used for data verification:

Input data twiceInput data twice Double data

entry

Double data entry

2.2 Data Control

Input Data Twice An operator inputs the data twice and allows the

computer system to check the second entry against the first one.

It reports any discrepancies and the operator is required to correct the error manually.

Example: When changing the password of your e-mail account, you

have to re-enter the password for confirmation.

2.2 Data Control

Input Data Twice

Input data twice

2.2 Data Control

Double Data Entry Two operators enter the same source document into two

different files. The computer compares the two files and reports any

discrepancies between them. The discrepancies must be checked and corrected

manually.

2.2 Data Control

Double Data Entry

Double data entry

1st operator

2nd operator

Check for discrepancies

2.2 Data Control

Double Data Entry

1st operator

2nd operator

Check for discrepancies

Double data entry

2.3 Data Hierarchy

The data hierarchy of a database system can be divided into 4 levels:

Field Record Table Database

2.3 Data Hierarchy

Database Hierarchy

Level Description Example of usage

Field

It is the smallest unit of data that can be accessed by a user.

Represent a specific fact of a

record. Can be of type: numeric,

text, date, Boolean, etc.

Telephone number Mailing address Monthly income Marital status

Record

It is a collection of related fields.

Contain information of one specific entity.

The data in a key field can be used to uniquely identify one

particular record from the others.

Data hierarchy

2.3 Data Hierarchy

Level Description Example of usage

Table

It is a collection of related records with identical record structures.

Contain the information of all entities stored in specific record format.

All records in a table have unique and different values in key field.

Database

It is a collection of related tables.

May link records from different tables dynamically with one or more key fields.

Data hierarchy

Database Hierarchy

Two tables can be linked with a key field.

2.3 Data Hierarchy

Database

Record

Field

Hierarchical structure of data

2.4 Database Management System (DBMS) DBMS is a computer program that provides functions to:

manage the database structure store, organize and retrieve data in a database

Application of DBMS Example

Database serverMySQL, Oracle Enterprise Manager, Microsoft SQL Server

Small office and personal use

Microsoft Access, Microsoft Visual FoxPro, FileMaker Pro

Examples of DBMS

2.4 Database Management System (DBMS)

Examples of database server

MySQL

Oracle Enterprise Manager

2.4 Database Management System (DBMS)

Examples of DBMS for personal use

Microsoft Access

Visual FoxPro

2.4 Database Management System (DBMS) Basic Functions to Organize Tables

Basic DBMS functions for organizing a table

Function Description Example

Create table

Create a new table and specify the record structure.

To create a table ‘student’ with fields ‘Name’, ‘ID’ and ‘Age’:

SQL commandCREATE TABLE student (Name char(24), ID char(8), Age decimal(3,0))Microsoft Access (in Design View)



Modify table

Change the structure of an existing table.

To remove the field ‘Age’ from table ‘student’:

SQL commandALTER TABLE student DROP Age

Microsoft Access (in Design View)




Delete table

Remove a table from a database. All records in the table will also be deleted permanently.

To remove table ‘student’:

SQL commandDROP TABLE student


2.4 Database Management System (DBMS) Basic Functions to Manage Records in a

Table

Basic DBMS functions for manipulating table records


Add a new record

Create a new record. Users are subsequently able to input information into it.

To add a new record:

SQL commandINSERT TABLE student VALUES ("Ada Lee", "92001", 15)Visual FoxProAPPEND


TableFunction Description Example

Delete existingrecords

Remove unwanted records from a table.Users can issue a delete command with a condition statement such that all records matching the condition will be deleted at one time.

To delete all records of students who are younger than 14-year old:

SQL commandDELETE FROM TABLE student WHERE Age < 14Visual FoxProUSE studentDELETE FOR Age < 14

Note: All the matched records are marked for deletion. They will be removed permanently by issuing the command PACK.Basic DBMS functions for manipulating table records



Modify existingrecords

Change the field values of selected records.Users can issue an updatecommand with a condition statement such that all records matching the condition will be changed at one time.

To add 1 to all records in the ‘Age’ column for students whose age are 16 or above:

SQL commandUPDATE TABLE student SET Age = Age + 1 WHERE Age >= 16Visual FoxProREPLACE Age WITH Age + 1 FOR Age >= 16




Browse records

Display all or part of records of a table in a window.Users may also be allowed to add, modify or delete records in the browse window.

To display all records in a browse window.

Microsoft Access

Visual FoxProUSE studentBROWSE

Double-click to open a browse window for the table ‘student’.


2.4 Database Management System (DBMS) Essential Features for Database

Management Provides features to:

facilitate the data input in data entry forms display results in query forms and generate reports

Filter Allows users to specify a filtering condition Only the records that satisfy the condition are accessible

in the database table. The unmatched records are hidden temporarily until the

filter constraint is released.


Management Filter

Table containing all records

Define a filter (e.g. logon is equal to "Y")on a table.

Filtering records from a table


Management Filter

Table shows records that satisfy the filtering condition.

Filtering records from a table


Management Sort

Change the order of records according to the value of one or more fields in a table.

The sorted records are usually saved in a new table.

Unsorted records in the table ‘Userlist’

Records are sorted by ‘Fullname’ and are stored in a new table ‘Sortedlist’.

Sorting records in a table

Visual FoxPro command:Sort to sortedliston fullname ascending


Management Sort

Allows records to be rearranged in a specific order permanently

A slow process if the number of records is large Index

A small file containing a number of index keys. The index keys are created and arranged according to

the index expression. One index file refers to one instance of a specific table

only. An index file must be re-indexed when values of the field

involved in the index expression have been changed. The index files are usually small relative to the referred

table. They can be loaded into the main memory for speeding

up the access process.


Management Index

A Visual FoxPro example showing how to access records by using an index file

Create an index file:Index on Fullname to user_index ascending

Unsorted records in the table ‘Userlist’

Records are accessed according to the order specified the index file.

Fullname Rec. No.

Joyce Wong 2

Ko Ko Lo 4

Mark White 3

Ricky Mok 1


Management Index

Records are accessed in the original order as stored in the table ‘Userlist’.

Stop using the index file:Set index to

A Visual FoxPro example showing how to access records by using an index file


Management Data Entry Forms

Can be treated as the interfaces for adding or modifying records

Advantages: Provide a user-friendly environment for data entry Perform data validation such as range check and type check of

test marks Perform data verification such as double data entry of renewal

password Simplify data entry with features like pull down menu, check

box and list box Modify related records from two or more tables in one form


Management Data Entry Forms

Data entry form of Microsoft Access


Management Query Forms

Provide an interface for users to extract and display records which match the specified conditions

The extracted data can further be modified, deleted or stored in a new table.

Define queries with selection criteria in the Design View.

Display selected records in query windowtemporarily.

Extracting data by using query form


Management Query Forms

Advantages: Can be stored and are reusable Able to show data from two or more tables Able to display calculated results using the extracted records

Writing commands in Structured Query Language (SQL) is another way to enquire about results from a database.SELECT bookname, type, price FROM book WHERE ((type = 'IT') or (type

= 'ENG'))


Management Reports

Used to display results on the computer screen or produce hard copies with a printer in the defined format

Advantages: Can be stored and are reusable Able to display data and calculated results from two or more

tables. Able to print results in specified locations, styles and formats

accurately


Management Reports

Creating a report in Microsoft Access

Click the ‘Report’ button.


Management Reports

Click the ‘Report’ button.

Creating a report in Microsoft Access

2.5 File Access Modes

The access mode used to access database records depends on which media the database resides in.

Sequential access mode Hard disk Optical disk Floppy disk

Direct access mode Magnetic tape


Using magnetic tapes to back up data in a large organization Using hard disks in a server

room


Sequential Access Mode Magnetic tape

Data records are stored one by one along the lengthy magnetic strip.

Need to be rewound forward or backward until the desired record is just underneath the read/write head before the reading or writing operation can proceed

The seek time is long and unpredictable when records are retrieved randomly.

Not a common medium for storing frequently used data records

Mainly used for backing up and recovering of database and data files in computer servers


Direct Access Mode Hard disk

The most common storage medium for secondary storage due to its high reliability, speed and capacity.

During read/write operations, the magnetic disk plate (platter) rotates and the read/write head moves across the platter surface at the same time to the location containing the data.

Once the target location is moved under the read/write head, the read/write operations can proceed.

The seek time is much shorter and predictable, ranging from 8 ms to 20 ms.

The most essential secondary storage device for a computer system


Direct Access Mode Hard disk

Read / write head movesacross platter surface to the right track and waits for the arrival of target data.

Target data

Platter surface rotates to move the data to the read / write head.

The working mechanism of a hard disk


Direct Access Mode Other examples:

Floppy disks DVD-RAM CD-RW

They are limited in capacity and speed of data access. They are rarely used as the storage media for database

systems and frequently used data files.

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