Final answer:
The keywords to indicate that NULL values should be listed first in SQL query results are NULLS FIRST and NULLS LAST. They are used in conjunction with the ORDER BY clause to sort data and control the placement of NULL values.
Step-by-step explanation:
The NULLS FIRST and NULLS LAST keywords can be included in an ORDER BY clause to indicate that NULL values should be listed first or last in the results, respectively. In SQL, the ORDER BY clause is used to sort data returned by a query in either ascending or descending order. The handling of NULL values is important because, by default, NULL values are treated differently by different database systems. Some systems list them at the beginning by default, whereas others list them at the end. By explicitly stating NULLS FIRST or NULLS LAST, you can control where and how these NULL values appear in your sorted data, ensuring consistent and predictable results.