NULL behavior in comparisons in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does NULL represent in SQL?

NULL represents a missing or unknown value in a database. It is not the same as zero or an empty string.

beginner

What is the result of comparing NULL with any value using = or <>?

The result is UNKNOWN, which behaves like FALSE in WHERE clauses. NULL = NULL does not return TRUE.

beginner

How do you properly check if a column value is NULL?

Use IS NULL or IS NOT NULL to check for NULL values instead of = or <>.

intermediate

What is the result of NULL OR TRUE in SQL?

The result is TRUE because TRUE in OR makes the whole expression TRUE regardless of NULL.

intermediate

Why does NULL cause three-valued logic in SQL?

Because NULL means unknown, comparisons can be TRUE, FALSE, or UNKNOWN, affecting query results and filters.

What does the expression NULL = NULL return in SQL?

A UNKNOWN

B FALSE

C TRUE

D NULL

NULL = NULL returns UNKNOWN because NULL means unknown value, so SQL cannot confirm equality.

Which SQL syntax correctly checks if a column 'age' is NULL?

A age = NULL

B age = ''

C age IS NULL

D age <> NULL

Use IS NULL to check for NULL values. = NULL or <> NULL do not work as expected.

What is the result of the condition NULL AND TRUE?

A UNKNOWN

B FALSE

C NULL

D TRUE

NULL AND TRUE returns UNKNOWN because NULL means unknown, so the result is unknown.

If a WHERE clause condition evaluates to UNKNOWN, what happens to that row?

A It is converted to TRUE

B It is included in the result

C It causes an error

D It is excluded from the result

Rows with conditions evaluating to UNKNOWN are excluded from query results.

Which logical operator can turn a NULL value into TRUE in an expression?

A AND

B OR

C NOT

D XOR

OR with TRUE returns TRUE even if the other operand is NULL.

Explain how NULL affects comparison operations in SQL and how to properly check for NULL values.

Think about why NULL = NULL is not TRUE.

Describe the three-valued logic in SQL caused by NULL and how it impacts query filtering.

Consider how NULL changes AND, OR, and NOT results.