G057 - Lecture 04

From Entity to Table

Mr C JohnstonICT Teacher

www.computechedu.co.uk

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.

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!

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)

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.

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.

Attribute Data Types Text or String:

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)

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

Attribute Data Types Currency

Offers more precision for storing prices Boolean / Logical

True, False On, Off Yes, No

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

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

StudentIDTitleFirst NameSecond NameCityIDPost CodeDOBApplicationDate

AutoNumberTextTextTextNumberTextDate/TimeDate/Time

Auto4chars15chars20charsLong Integer8charsShort dateShort date

Topic Exercise1) For each of the scenarios last week identify attributes

for each entity and produce a Data Dictionary.2) Compile both the ERD and DD into single page LDM

for each scenario.3) 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.

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

Methods of Validation Format check

Usually controlled by the data type and size selectede.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 somethinge.g. [surname] - required = yes, [email] – required = no

Range check What are the range of numerical values allowed to be enterede.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. 6754 is calculated by: 365 days a year * 18 years +

4 extra days for leap years

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 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) 000000 (allows 02476 123123 and 07957

123123)

List of valid values Limit the values to only those on a defined list of from another tablee.g. [gender] - “male” or “female”, [carID] – looked up from CAR

Default values Used to suggest a value for a field which is common or defaulte.g. [city] – default = coventry

[orderDateTime] – default = now()

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: http://office.microsoft.com/en-us/access-help/control-data-entry-formats-with-input-masks-HA010096452.aspx

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

StudentIDTitle

First NameSecond NameCityID

Post CodeDOB

ApplicationDate

AutoNumberText

TextTextNumber

TextDate/Time

Date/Time

Auto4chars

15chars20charsLong Integer

8charsShort date

Short date

Required = yesRequired = yes“Mr” OR “Mrs” OR “Miss” OR “Ms”Required = yesRequired = yesRequired = yesLooked up in [city]Required = yesRequired = yes<=date()-5478Required = yesDefault Value = now()>=Date()

Validation Text – a friendly message which alters the user if a validation method has been broken and tips on

how to solve the error

Topic Exercise1) For each of the data dictionaries you have created

add some validation rules to each entity.