The Data Dictionary
We have now created the 3NF descriptions of our data. We called these descriptions Entities. However if we are going to implement them in a Relational Database we usually call them Tables as you will recall from earlier. The main reason we call them Entities at all is that they may be implemented in a variety of applications including programming languages such as Java, Visual Basic or C# and these languages use different terms for the implementations. Regardless of what package you use we will have to tell it some more information about the entity so that it can be implemented properly. The typical information we usually have to specify for each attribute is as follows:
- Type refers to the data the attribute will hold. This can be numeric, text, currency, date, for example although there are many others.
- Size refers to the number of characters required for the attribute. For example a National Insurance Number is 9 characters long.
- Range refers to the allowable set of values. For example our range of valid Product-Codes may be AA001-ZZ999.
- Comments refers to any relevant information we wish to add.
The collection of entities we are going to implement along with each one's list of attributes with all the relevant information is called the Data Dictionary. This is the stage directly before implementation. This is usually recorded on a special document. Click on the link below to see an example of this using the entity Student:
This shows the entity Student. It has four attributes. For example the Student Number attribute is text, size 6 bytes, valid range is 900001 to 999999. There are no comments for any of the attributes.
- Complete the Data Dictionary entries for the other entities in Mosspark Community College case study.
Complete the Data Dictionary entries for the Mosspark Surgery case study.
In the next section we will create the tables in Microsoft Access and learn how to enter the data into the table.