Database Design Concepts INFO1408 Term 2 week 1 Data validation and Referential integrity

Objectives To identify the need for validation and verification Identify 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 codes Phone numbers Starting names with capital letters Number of digits after the decimal point Dates (eg 3/2/04 = 3 feb 2004)

Input masks This is a format that is applied when users enter data An 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 (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 spacers Caption ( name of field to appear on reports etc) Default value value that is automatically entered- user must overtype if different- set to today ’ s date Validation rule – date entered must be before or equal to today ’ s date Validation text- error message to be displayed if incorrect date entered Required - 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 date This 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 tables It 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 S1FrodoBag EndC1 S2ElrondRivendellC1 S3SamBag EndC1 Course NoCourse Name C1 C2 BIT Computing

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 ways If 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 user Two 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 lecture Validation ( field and record level) Formatting and input masks Constraints Primary key constraints ( uniqueness) Referential constraints ( foreign Keys Integrity constraints ( not null and check constraints)

References Nat Mcbride- Introduction to Access 2000