W3. SQL Null Values

Question 1

Q: What is a NULL value in SQL?

Answer 1

A: A field with no value, meaning it was left blank during record creation.

Question 2

Q: How is a NULL value different from a zero or a space?

Answer 2

A: NULL represents an absence of value, while zero and space are actual values.

Question 3

Q: How do you test for NULL values in SQL?

Answer 3

A: Use the IS NULL or IS NOT NULL operators.

Question 4

Q: What is the syntax for using the IS NULL operator?

Answer 4

SELECT column_names
FROM table_name
WHERE column_name IS NULL;

Question 5

Q: Write a query to select customers with a NULL value in the “Address” field.

Answer 5

SELECT CustomerName, ContactName, Address 
FROM Customers 
WHERE Address IS NULL;

Question 6

Q: Why can’t you use = or <> to test for NULL values?

Answer 6

A: Because NULL represents an absence of value and cannot be compared directly with comparison operators.

Question 7

Q: What does the IS NOT NULL operator do?

Answer 7

A: It tests for non-empty values, returning records where the specified field is not NULL.

Question 8

Q: Write a query to select customers with a value in the “Address” field.

Answer 8

SELECT CustomerName, ContactName, Address 
FROM Customers 
WHERE Address IS NOT NULL;