InterBase Quick Start: Part IV - Testing for an Unknown Value

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Another type of comparison tests for the absence or presence of a value. Use the IS NULL operator to test whether a value is unknown. To test for the presence of any value, use IS NOT NULL.

Testing for NULL

Execute the following query to retrieve the names of employees who do not have phone extensions:
```
SELECT last_name,
       first_name,
       phone_ext
FROM   Employee
WHERE  phone_ext IS NULL
```
The image below shows the expected result:
Now execute the statement using IS NOT NULL to retrieve the names of employees who do have phone extensions:
```
SELECT last_name,
       first_name,
       phone_ext
FROM   Employee
WHERE  phone_ext IS NOT NULL
```
The image below shows the expected result:

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Comparing Against a Range or List of Values