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Tutorial Exercise 3

Creating Distinct Data Types and Casts

This exercise demonstrates how to create distinct data types and the casts that convert them.

The Mercury DataBlade module contains distinct data types to represent Fahrenheit and Celsius temperatures and casts to convert between the two.

You create the distinct data type hierarchy, beginning with the temperature distinct data type, which is based on the built-in double-precision data type.

Then you create the CTemp and FTemp types that inherit from temperature, as shown in the following diagram.

The Mercury DataBlade module ensures that users cannot enter temperatures below absolute zero in the CTemp and FTemp types. If a user attempts to insert an incorrect value, the INSERT statement fails and returns an error. The DataBlade module uses user-defined support routines and casts to check the input value.

Using Casts

The Mercury DataBlade module uses casts in two ways:

• To convert temperatures between the Celsius and Fahrenheit.
• To enable users to enter numbers as integer and decimal data types directly into CTemp and FTemp columns without having to use an explicit cast in their SQL statements.

The temperature data type is implemented to reduce the number of casts you need to define. If you defined casts from integer and decimal for both CTemp and FTemp, you would create a total of six casts. Instead, you add integer and decimal casts only to the temperature type; the CTemp and FTemp types inherit the casts. This approach reduces the number of casts you have to define from six to four.

DataBlade Module Objects

The Mercury DataBlade module creates the following user-defined objects.

Tutorial Steps

This exercise consists of 16 steps. It takes approximately one hour to complete.

Start BladeSmith from the Informix program group and choose Project New.

Enter the following information into the New Project wizard.

Click Finish.

Choose Project Save; create a new directory called Mercury in a convenient place on your hard drive and save your project in this directory as a file called Mercury.ibs.

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