Examine the description or the BOOKS_TRANSACTIONS table:FOR customers whose income level has a value, you...

D: True. This query selects the first name and calculates the due amount as 5% of the credit limit from the customers table where the income level is not null and the credit limit is also not null. The IS NOT NULL operator is used to check for non-null values correctly.

The IS NOT NULL operator is the correct way to check for non-null values in SQL. The <> NULL and != NULL comparisons are incorrect because NULL is not a value that can be compared using these operators; it represents the absence of a value.

There is no due_amount column in the customers table according to the structure provided, so any WHERE clause referencing due_amount is irrelevant and incorrect. The due_amount is a derived column in the SELECT clause.

The correct syntax to check for non-null values is IS NOT NULL, which is used in the WHERE clause of the correct answer.

References:

The use of IS NOT NULL in the WHERE clause to filter out null values is documented in Oracle's SQL reference.

NULL comparisons must be done with IS NULL or IS NOT NULL for correct logical evaluation in SQL.