Why equals NULL fails in SQL - Quick Recap

Recall & Review

beginner

Why does the expression column = NULL fail to return expected results in SQL?

Because NULL means unknown or missing value, and comparing anything to NULL with = does not return true or false but unknown, so the condition fails.

beginner

What is the correct way to check if a column value is NULL in SQL?

Use IS NULL or IS NOT NULL instead of = NULL or <> NULL.

beginner

What does NULL represent in a database?

NULL represents a missing, unknown, or undefined value, not zero or empty string.

intermediate

Why does NULL = NULL return false or unknown in SQL?

Because NULL means unknown, comparing two unknowns does not guarantee equality, so the result is unknown, not true.

intermediate

How does SQL treat logical operations involving NULL?

SQL uses three-valued logic: true, false, and unknown. Any comparison with NULL results in unknown unless handled with IS NULL or IS NOT NULL.

Which SQL expression correctly checks if a column age is NULL?

A age == NULL

B age = NULL

C age IS NULL

D age != NULL

The correct way to check for NULL is using IS NULL. Using = NULL does not work because NULL means unknown.

What does NULL mean in SQL?

A Zero value

B Unknown or missing value

C Empty string

D False boolean

NULL means the value is unknown or missing, not zero or empty.

What is the result of NULL = NULL in SQL?

A Unknown

B False

C Error

D True

Comparing NULL to NULL results in unknown because NULL means unknown value.

Which logical system does SQL use when dealing with NULL?

A Three-valued logic

B Two-valued logic

C Boolean algebra

D Fuzzy logic

SQL uses three-valued logic: true, false, and unknown (NULL).

If you want to find rows where a column is NOT NULL, which is correct?

A column != NULL

B column <> NULL

C column = NOT NULL

D column IS NOT NULL

Use IS NOT NULL to check for values that are not NULL.

Explain why using = NULL in SQL does not work as expected.

Think about how SQL treats unknown values in comparisons.

Describe how SQL handles logical operations involving NULL values.

Consider the difference between NULL and normal values.