1. Mr C Johnston ICT Teacher www.computechedu.co.uk
G057 - Lecture 04 From Entity to Table
Mr C Johnston
ICT Teacher

2. Session Objectives Revise terms from last session,
Understand how to construct database tables,
Understand that each attribute needs a data-type,
Know different types of validation rules which can be applied to attributes to ensure that data remains of the correct type,
Know correct methods of documenting tables within a database and its attributes.

3. Revision of Terms An entity is a person, place or object
More commonly in English its know as a noun
E.g a student at college could be an entity
Entities have attributes
E.g a student has a name, address, phone number etc.
Each entity needs a unique identifier if it is to be used in a relational database – this is known as a primary key in database land!

4. Revision of Terms Entities have relationships with one another
There are three types of relationships
One to One, One to Many, Many to Many
One to Many relationships are the only type allowed - the other two types need resolving
Relationships between entities are shown in an Entity Relationship Diagram (ERD)
This forms one component of the Logical Data Model (LDM)

5. Constructing Database Tables
A database table is made up of a set of entities
Each database table needs a name
We normally use the name given to the entity
It should be written in capitals
Tables need attributes, which are found during the process of normalisation
Each attribute needs a data type and some form of validation.

6. Attribute Data Types
Attributes must be one of the following data types:
Text or String,
Numeric,
Date and Time,
Currency,
Boolean or Logical,
AutoNumber.
Data types have different sizes to choose from – you need to pick an appropriate size as well as data type for each attribute,
Some common sizes for each data type are given on the next few slides.

7. Attribute Data Types Text or String: Number:
Consists of a sequence of any characters,
Limited to 255 characters,
For longer text requirements you can use a memo data type.
Number:
Byte: 0 to 255
Integer: -32,768 to 32,768
Long Integer: 2,147,483,648 to 2,147,483,647
Single: small decimal numbers (7dp)
Double: medium decimal numbers (15dp)
Decimal: large decimal numbers (28dp)

8. Attribute Data Types Date and Time General Date – 19/06/1994 17:34:23
Long Date – 19 June 1994
Medium Date – 19 – JUN - 94
Short Date – 19/06/1994
Long Time – 17:34:23
Medium Time – 05:34 PM
Short Time – 17:34

9. Attribute Data Types Currency Boolean / Logical
Offers more precision for storing prices
Boolean / Logical
True, False
On, Off
Yes, No

10. Using The Correct Data Types
You must ensure that the correct data type is chosen for an attribute
The larger the maximum value the more memory in the database is taken up
E.g phone numbers don’t need to be calculated with so can be stored as text

11. The Data Dictionary
The data dictionary (DD) is a table used to document entities,
A data dictionary along with an ERD makes up a logical data model (LDM)
A data dictionary for a student may look like:
Entity
Key
Attribute
Data type
Size
Validation
STUDENT
Primary
Foreign
StudentID
Title
First Name
Second Name
CityID
Post Code
DOB
ApplicationDate
AutoNumber
Text
Number
Date/Time
Auto
4chars
15chars
20chars
Long Integer
8chars
Short date

12. Topic Exercise
For each of the scenarios last week identify attributes for each entity and produce a Data Dictionary.
Compile both the ERD and DD into single page LDM for each scenario.
Create a LDM for customers wishing to book a ferry crossing. Each crossing runs serveral times a time and has one ferry assigned to it. A ferry could be responsible for more than one crossing.

13. Ensuring Data Correctness
Keeping data integral is part of the job which the DBMS plays
There are two methods to do this:
Validation
Ensuring data is within the right context
Access provides lots of methods for this
Verification
Ensuring data is correct
Access doesn’t provide any methods for this

14. Methods of Validation Format check Presence check Range check
Now() is a function which gives the current system date and time… useful for constructing validation rules, queries, forms and reports in Access.
Other useful functions include: Date() which only gives the date and Time() which just gives the time.
Methods of Validation
Format check
Usually controlled by the data type and size selected
e.g. If [price] field was set as an integer it would not work properly as the prices entered would have to be whole numbers!!
Presence check
Is the field allowed to be left empty or must the user enter something
e.g. [surname] - required = yes, [ ] – required = no
Range check
What are the range of numerical values allowed to be entered
e.g. [price] - >= 0 AND <=1000,
[order] >= Now()
If customers have to be over 18:
[DOB] <= (Date()-6754)
6754 is number of days old you would be if 18. So we take this away from current date to check the DOB is allowed is calculated by: 365 days a year * 18 years + 4 extra days for leap years

15. Methods of Validation Picture check / Input mask List of valid values
Constructing input masks can be tricky – L is for mandatory letters (A-Z), ? is for optional letter (A-Z), 0 is for mandatory numbers (0-9), 9 is for optional numbers (0-9). For full details see:
Methods of Validation
Picture check / Input mask
Used to check if the data entered looks like a defined pattern of text and numerical characters. Suitable for post codes (can be awkward) and phone numbers
e.g. [postCode] – LL0 0LL (allows CV1 5NL but B1 5NL and CV10 5NL not allowed!!)
[phoneNumber] – (00000) (allows and )
List of valid values
Limit the values to only those on a defined list of from another table
e.g. [gender] - “male” or “female”, [carID] – looked up from CAR
Default values
Used to suggest a value for a field which is common or default
e.g. [city] – default = coventry
[orderDateTime] – default = now()

16. Data Dictionary With Validation
Validation Text – a friendly message which alters the user if a validation method has been broken and tips on how to solve the error
Data Dictionary With Validation
For each field within your data dictionary think about which validation rule could be added – remember to add a validation text column to record any planned validation text.
Entity
Key
Attribute
Data type
Size
Validation
STUDENT
Primary
Foreign
StudentID
Title
First Name
Second Name
CityID
Post Code
DOB
ApplicationDate
AutoNumber
Text
Number
Date/Time
Auto
4chars
15chars
20chars
Long Integer
8chars
Short date
Required = yes
“Mr” OR “Mrs” OR “Miss” OR “Ms”
Looked up in [city]
<=date()-5478
Default Value = now()
>=Date()

17. Topic Exercise
For each of the data dictionaries you have created add some validation rules to each entity.