Null handling

• NULL is not a value indicating a missing entry, but a placeholder for anything possible,
• NULL = NULL evaluates to NULL, since one cannot know if one unknown is equal to another unknown,
• NULL <> NULL evaluates to NULL, using same logic,
• DISTINCT column may return multiple NULL values.

• null == null evaluates to true,
• null != null evaluates to false,
• distinct column cannot contain multiple values.

PRQL

SQL

SELECT
FROM
  employees
WHERE
  first_name IS NULL
  AND last_name IS NOT NULL