Example Database Use of term: object Fieldsparamesh/COMP1000/Ch2-TableDesign-Que… · “All organizations … are required to establish and maintain an audit committee that shall

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Copyright © 2008 Pearson Prentice

Hall. All rights reserved.

Relational Databases and Multi-Table Queries

DESIGNING TABLES AND

QUERIES

Example Database

• Inventory

Part No. Description Quantity Unit Price

P001 Widget 10 $1.50

P002 Gadget 12 $10.20

P201 Object 25 $0.12

Example Database • Student database

ID Name Faculty Degree Credit

Points

1234567 Jane Doe Science MSc 50

7654321 John Smith Arts BA 48

Fields

• Split that description of the object into a number of

attributes or fields. • Each field should have a simple and unambiguous

meaning. • The way in which fields are chosen should be guided

by an understanding of how the data is to be manipulated in the database.

• There is no single perfect way to do this; some art is involved.

• Some designs, however, are better than others.

John works for Peter 8 hrs a

day.

Use of term: object – not

ACCESS object.

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Table Design Considerations – Store Data in its Smallest part

• For greater flexibility, store data in its smallest part – Instead of one field for an address, use many. – Instead of one field for a name, two or three.

Like this

Not like this

Field Examples – Bad

• There is nothing to prevent you from placing an entire body of data in a single field of a single record

Field Examples – Bad

• There is nothing to prevent you from placing each letter in a separate field

Fields

• Every field in Access has a data type (e.g., numeric, text, hyperlink, etc.)

• Giving a field a type restricts the data that can be entered into that field.

– Advantage: Information that does not conform to the data type cannot be entered into the database.

• This is one way to maintain integrity of data.

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Field Data Types

• Autonumber - MS Access assigns unique id to each record

• Numeric

• Currency

• Yes/No - Boolean: true/false; on/off values

• Text - 255 characters (max)

• Memo - 65 535 chars (max)

• Date/time

• OLE (Object Linking and Embedding) object, eg. image; objects from other Microsoft applications or pictures

• Hyperlink – URL

Why Enforce Data Types?

• Prevent data entry errors.

• Reduce amount of storage space required to store database.

– For example: If we know the object is boolean, allocation is 1 byte (8 bits); if it is autonumber, it is long integer; that is, allocation is 4 bytes(32 bits).

Calculated Fields • Calculated field is a field we may use to hold some

important values computed from other fields.

• TotalPrice below is a calculated field.


Hall. All rights reserved. 11

Part

No.

Descrip

tion

Quant

ity

Unit

Price

P001 Widget 10 $1.50

P002 Gadget 12 $10.20

P201 Object 25 $0.12

TotalPrice =

quantity *

UnitPrice



Table Design Considerations

• Avoid Calculated Fields in Tables.

• Calculated fields should be used mainly in queries and reports.

Calculated fields

in a query

Calculated field in a report

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Design view shows:

- Fields; and their

- Properties.

Properties of Fields



Table Design Considerations: Field Properties Setting Field Size Property

• Example above shows setting the field size (done in Table Design View)

• Always anticipate the current field size may one day need to be larger

Setting field size to 10. For text: 0 – 255 chars. (default: 50 chars) For Autonumber: Long integer. –2,147,483,648 to 2,147,483,647

For Integer: –32,768 to 32,767 (Obtained by selecting a field property and pressing function key F1)



Table Design Considerations – Field Properties

Validation Rules

• Used to avoid data entry errors by restricting what can be entered

• Validation text can be used to provide an explanation of the type of data that is allowed in a field

Set validation rules in the Field Properties grid of Table Design View Examples: < > 0 Entry must be a nonzero value.

> 1000 Or Is Null Entry must be blank or greater than 1000.



Table Design Consideration - Plan for Date Arithmetic

• Using a data type of date/time for all date fields allows the use of date arithmetic

Fields declared

as a data type

of Date/Time

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Table Design Consideration – Sarbanes Oxley Act (SOX)

• Protects the general public and companies shareholders against fraudulent practices and accounting errors.

– Requires that all business records be saved for a period of fie years and be made available to the SEC (Securities and Exchange Commission) upon request

“All organizations … are required to establish and maintain an audit committee that shall

be responsible for appointment of any registerd public accounting firm ...”

Section 802(a) of the SOX, 18 U.S.C. § 1519 states:

“Whoever knowingly alters, destroys, mutilates, conceals, covers up, falsifies, or makes a

false entry in any record, document, or tangible object with the intent to impede, obstruct,

or influence the investigation or proper administration of any matter within the jurisdiction

of any department or agency of the United States or any case filed under title 11, or in

relation to or contemplation of any such matter or case, shall be fined under this title,

imprisoned not more than 20 years, or both. ”



Table Design Considerations - PNPI

Short for personal, non-public, information

PNPI laws govern the safeguarding of data such as:

social security numbers

credit card numbers

bank account numbers

medical records

Some ways of protecting PNPI

Encrypt Electronic Transmissions

Do Not Store PNPI Locally

Ensure PNPI Security When Working from

Home or Outside the University

Have Computer Equipment Audited



Table Design Considerations – Design Multiple Tables

• Using multiple tables helps reduce redundancy

– The process is also referred to as normalization

Multiple table tabs identify open tables

Multiple tables shown in the Navigation pane



NORMALIZATION: Problems with single table

Redundancy problem: Same information appears multiple times. Update

can lead to inconsistencies.

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o It may not be possible to enter partial data.

o Partial deletion of a record may not be possible.

NORMALIZATION: Problems with single table



Referential Integrity

Consider the tables Customers and Orders.

1. We cannot add any record in the Orders table unless its customerID is valid in the Customers table.

2. If the primary key for a record in the Customers table changes, all corresponding records in the

Orders table must be modified .

3. If a record in the Customers table is deleted, all corresponding records in the Orders table must be

deleted .



Creating Tables – From the Create Tab • Enter table data directly in fields

– From the Create Tab, click Table • Use a table template

– From the Create Tab, click Table Templates

Enter data directly into a table,

including the field names

Enter field names, data

types and descriptions in

Table Design View

Begin with a template Copyright © 2008 Pearson Prentice


Creating Tables – From the Import Tab

• Click the application from which to import or • Choose the type of file you wish to import

Click the appropriate application

button Choose a file type to import

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Create Tables – Specifying field names

• After choosing your method of creation begin implementing the table design – Use CamelCase notation for field names – Specify data types – Establish a primary key – Consider the need for a foreign key

Table View

Table Design View

Add field in Table View



Create Tables – Primary Key

• Tables are automatically created with an AutoNumber field which serves as the primary key

• To change the primary key – Select a field in Design View

– Click the primary key icon

Primary Key Field Primary Key icon



Create Tables – Consider a Foreign Key

• Based on the above example: – Customer Id is the foreign key in the Orders Table – This is referred to as a One to Many Relationship

Primary Key Customer ID –

occurring as a Primary Key in Customer Table

Customer ID –will only appear in one record; there must only be one unique id per customer

Foreign Key Customer ID –

occurring as a Regular Field in Orders Table

Customer ID may appear many times – one customer can place many orders



Working with Multiple Tables – Table Relationships

• The strength of Access is the fact that it is a relational database – This means you can have multiple tables and

create relationships between each table

– This helps eliminate redundant data

Relationship between two tables

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Working with Multiple Tables – Referential Integrity

• Assures that the references to relationships between data is accurate

• Established when creating the relationship between two tables

Enforce Referential Integrity



Working with Multiple tables - Cascades

• When active, data changed in one table (that is in a relationship) will be changed in its related tables.

• Can be set when establishing relationships between tables.

Cascade update and cascade delete



Indexing

• Index properties can be set in the Field Properties pane of table Design View

• Relates the field values to the records that contain the field value

• Helps speed up some sorting and searching processes; but can slow down updation.

Indexed Property



Sharing Data with Excel

• Data can be imported from Excel

– It may be appended to an existing table

– It may be used to create a new table

Excel icon

External Data tab

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Sharing Data with Excel

• Select the Excel file you would like to import

• Select how you would like to import the data

– Appended – added to the end of an existing table

– New table – creates a new table in a database

– Linked – create a new table that is linked to the source file in Excel

Select the Source

Select the destination



Establishing Relationships

• Click the Database tools and click the

Relationships icon • In the Relationship window, click and drag a

field name from one table to a field name in a related table

Relationships icon Click and drag to create a relationship



Establishing Relationships

• Enter the appropriate settings in the Edit relationships dialog box and click Create

• A join line will appear when one table is joined to another

Infinity symbol notes referential integrity has been applied

Set referential integrity and cascades



Queries

• Queries allow us to ask questions about data

• This record set that answers our question is called a dataset

Employees table

Dataset resulting from querying table for only employees who are Sales Representatives

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Using Query Design View

• Query Design grid has two panes – the tables pane and the design pane

• Striking the F6 key will toggle you between sections

Table pane

Design

pane



Select Query

• Searches associated tables and returns a dataset that matches the query parameters

• Changes made to the dataset will be reflected in the associated tables



Specifying Criteria in a Select Query

• Field row – displays the field name

• Sort row – enables you to sort the dataset

• Show row – controls whether or not you see a field in the dataset

• Criteria row – determines the records that will be selected for display

Fields in design grid allow us to specify criteria for the dataset



Specifying Criteria – Currency and Operands

• Specify criteria with currency – Without the dollar sign – With or without the decimal point

• Use operands such as: – Less than and greater than – Equal to or not equal to

Greater than (>) operand

Currency amount entered without dollar sign

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Specifying Criteria – Wildcards

• Asterisk - searches for a pattern that includes any number of characters in the position of the asterisk

• Question mark - searches for a pattern that includes a single character in the position of the question mark

Query with asterisk wildcard and resulting dataset

Query with question mark and asterisk wildcard and resulting dataset to specify criteria for the dataset



Specifying Criteria – Null Values

• IS NULL finds only records that have no value

• IS NOT NULL excludes Null value records

Is Null criteria and resulting dataset

IS NOT NULL criteria and partial resulting dataset



Specifying Criteria – And and Or

• OR finds records that can match one or more conditions

• AND finds records that must match all criteria specified

Or Criterion and resulting

dataset

And criterion and resulting

dataset



Copy a Query

• Right click on the query - chose Copy form the shortcut menu

• Right click and chose paste

• In the Paste as dialog box, give the query a new name

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Run a Query

• Running, or executing, a query is done by clicking the Run command

Run command



Creating Queries – Using the Query Wizard

• From the Create tab, choose Query Wizard for the Other group

• Choose query type from the New Query dialog box

Query Wizard icon

Select Simple Query Wizard



Creating Queries – Using the Query Wizard: continued

• Select the Table/Queries to include and choose the desired fields

• Select aggregate totals needed in the Summary Options box



Creating Queries – Using the Query Wizard: continued

• Title your query and open in Datasheet View or Query Design View

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Understand Large Database Differences

• Most large companies separate their database into front and back ends

– Front end – contains the objects needed to interact with data, but not the tables where the record values reside

– Back end – contains the tables where the data resides

Documents

Example Database Use of term: object Fieldsparamesh/COMP1000/Ch2-TableDesign-Que… · “All organizations … are required to establish and maintain an audit committee that shall