Which statement about Null Values is true?

Which statement about Null Values is true?

• A. Null Values indicate data is unknown, missing, or inapplicable
• B. Null Values are always numeric zeros
• C. Null Values are the same as empty strings
• D. Null Values cannot be stored in any column

Answer: (A)

Null values indicate data that is unknown, missing, or not applicable. They are not the same as zero or an empty string, so treating them as such can lead to incorrect results in queries and calculations. In SQL, a NULL is a special marker inside a column and participates in three-valued logic: comparisons with NULL don’t yield true or false, they yield unknown. To find missing values you use IS NULL or IS NOT NULL, not ordinary equality checks. For example, checking whether a middle name exists requires middle_name IS NULL rather than middle_name = NULL. When you want to replace missing data with a default, you can use functions like COALESCE to supply a substitute value. Columns can store NULL values if they are defined to allow NULLs; primary keys and other constraints like NOT NULL prevent NULLs. So the statement that null values indicate data is unknown, missing, or inapplicable is the true description.

Null values indicate data that is unknown, missing, or not applicable. They are not the same as zero or an empty string, so treating them as such can lead to incorrect results in queries and calculations. In SQL, a NULL is a special marker inside a column and participates in three-valued logic: comparisons with NULL don’t yield true or false, they yield unknown. To find missing values you use IS NULL or IS NOT NULL, not ordinary equality checks. For example, checking whether a middle name exists requires middle_name IS NULL rather than middle_name = NULL. When you want to replace missing data with a default, you can use functions like COALESCE to supply a substitute value. Columns can store NULL values if they are defined to allow NULLs; primary keys and other constraints like NOT NULL prevent NULLs. So the statement that null values indicate data is unknown, missing, or inapplicable is the true description.