Database Design ConceptsINFO1408

Term 2 week 1Data validation and Referential integrity

Objectives

To identify the need for validation and verificationIdentify some of the built in constraints used in databases

Data validation & formatting

When data is entered into a database it is important that it is entered consistently- this ensures that we can set up queries and macros for the users as easily as possible.

Imagine a query that needed to work on all the possible ways someone could enter a date.

in other words it must be entered in the same format.

If you design and set up a database and enter all the data yourself this may not be a problem however this is unlikely to be the case in many organisations.

Formatting Data

Formatting is the term used to control the appearance of your data.

This formatting can also speed up data entry by filling in attribute values with default values such as today’s date or from other data already in the database.

As the designer you need to decide where formatting can be used.

Examples

Post codesPhone numbersStarting names with capital lettersNumber of digits after the decimal pointDates (eg 3/2/04 = 3 feb 2004)

Input masks

This is a format that is applied when users enter dataAn input mask will ensure your user inputs data in the correct format.

E.G Post code 1 or 2 letters (capitals) followed by up to 2

digits, a space, 1 digit then two letters (capital)

LE1 4RT or LE11 5TY or L1 4GH

Validation rules 

You can set validation rules for an attribute. This ensures that values entered into the attribute are appropriate for that attribute.

E.g. Date of birth to be before today’s date Members date of birth to be before 31.3.94

(i.e. less than10 years old) Price of a product to be greater than £0 Salary to be less than £30,000

Setting up validation & formatting

All of these are set up in Access for each attribute using the attribute property table

Format – short date Input mask / as spacersCaption ( name of field to appear on reports etc)Default value value that is automatically entered- user must overtype if different- set to today’s dateValidation rule –date entered must be before or equal to today’s dateValidation text- error message to be displayed if incorrect date enteredRequired - yes means a value must be entered

Caption

Default date today’s date date

in short date format

In put mask shows the / as spacers

Adding a new record to the table

If I try and remove the date and don’t put in a new date the required property comes into effect.

I cant move to the next attribute until I have entered another date. I am going to add tomorrows date ( 27/10/04)

This date does not agree with the validation rule. The Validation text is shown as an error message.

Record level validation

Sometimes you want to check that the value of an attribute compares with another attribute.

E.G End date of agreement must be after start date.

Set up record level validation by opening the table in design view and selecting view for the main menu followed by properties

This validation rule checks that the end date is after the start dateThis check is carried out when you move to the next record ( or row)

Constraints

Primary Key Constraints

The database will ensure that no row is added to a table that already has the same value for the key.

For composite keys it is possible for one of the columns to contain duplicate values in the table but the combination of values must be unique

Referential Integrity Constraints

Referential constraints are enforced using something called foreign keys.A foreign key is a key field of a table found in another table as a foreign key.This allows links to be made between the tablesIt allows us to reference other tables by enforcing the relationships between the entities

This arrangement tells us there is a relationship between student and course. The foreign key sets up the relationship

Student Number

Student Name

Student Address

Course No

S1 Frodo Bag End C1

S2 Elrond Rivendell C1

S3 Sam Bag End C1

Course No

Course Name

C1C2

BITComputing

Referential integrity

The DBMS will check that when ever a value is entered into a foreign key there is a corresponding row in the table which contains the same value

i.e. If we enter a course number in the student table there will be a row in the course table with that course number.

This works in several waysIf we enter a new row in the student table the DBMS will check there is a corresponding record for the course no in the course table otherwise it will reject it.If we update the value of course no in the student table the DBMS will check there is a value for the course no in the course table otherwise it will reject it.When we try and delete a record from the course table it will reject the delete,If we are sure we wish the related records to be deleted we can set it up automatically to delete all the records in the student table which have that same value of course no – known as a cascade delete.

Using an example The relationship is between Classes and Tutor

See week 2 lab on how to add this in Access

Integrity constraints

These are constraints to help the accuracy of the database- There is no way round them for the userTwo types Not null – There must be a value for

this attribute Check constraint - uses a simple

logic statement to work out whether the value of an attribute is acceptable

Example of not null

constraint

Example of check constraint

Summary

We have covered in this lectureValidation ( field and record level)Formatting and input masksConstraints Primary key constraints ( uniqueness) Referential constraints ( foreign Keys Integrity constraints ( not null and

check constraints)

References

Nat Mcbride- Introduction to Access 2000