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.

Practice

1. Why does the SQL condition column = NULL always fail to find rows with NULL values?

easy

A. Because NULL means unknown and cannot be compared with = or != operators

B. Because NULL is treated as zero in SQL

C. Because NULL is a string and needs quotes

D. Because NULL values are automatically excluded from queries

Solution

1. Step 1: Understand NULL meaning in SQL

   NULL represents an unknown or missing value, not a specific value.

2. Step 2: Explain comparison behavior with NULL

   Comparing anything to NULL with = or != always returns false because NULL is unknown.

3. Final Answer:

   Because NULL means unknown and cannot be compared with = or != operators -> Option A

4. Quick Check:

   NULL comparison with = fails [OK]

Hint: Use IS NULL to check for NULL, not = NULL [OK]

Common Mistakes:

• Using = NULL to find NULL values
• Assuming NULL equals zero
• Trying to quote NULL as a string

2. Which of the following SQL WHERE clauses correctly checks for NULL values in the age column?

easy

A. WHERE age != NULL

B. WHERE age = NULL

C. WHERE age IS NULL

D. WHERE age = 'NULL'

Solution

1. Step 1: Review correct syntax to check NULL

   SQL uses IS NULL to test if a column has NULL values.

2. Step 2: Analyze each option

   WHERE age = NULL and C use = or != with NULL which always fail; WHERE age = 'NULL' treats NULL as a string which is incorrect.

3. Final Answer:

   WHERE age IS NULL -> Option C

4. Quick Check:

   Use IS NULL for NULL checks [OK]

Hint: Always use IS NULL or IS NOT NULL for NULL checks [OK]

Common Mistakes:

• Using = NULL or != NULL in WHERE clause
• Quoting NULL as a string
• Confusing NULL with empty string

3. Given a table users with a column email that contains some NULL values, what will this query return?

SELECT * FROM users WHERE email = NULL;

medium

A. All rows where email is NULL

B. All rows where email is not NULL

C. Syntax error

D. No rows at all

Solution

1. Step 1: Understand the effect of = NULL in WHERE clause

   Comparing with = NULL always yields unknown (false), so no rows match.

2. Step 2: Predict query result

   The query returns zero rows because no row satisfies email = NULL.

3. Final Answer:

   No rows at all -> Option D

4. Quick Check:

   email = NULL returns no rows [OK]

Hint: = NULL never matches any row, returns empty result [OK]

Common Mistakes:

• Expecting rows with NULL to be returned
• Thinking = NULL is valid syntax
• Confusing NULL with empty string

4. You wrote this query to find rows with NULL in salary:

SELECT * FROM employees WHERE salary = NULL;

But it returns no rows. What is the fix?

medium

A. Use WHERE salary != NULL

B. Change to WHERE salary IS NULL

C. Add quotes: WHERE salary = 'NULL'

D. Change to WHERE salary = 0

Solution

1. Step 1: Identify why salary = NULL fails

   Comparing with = NULL always returns false, so no rows match.

2. Step 2: Use correct syntax to check NULL

   Use IS NULL to find rows where salary is NULL.

3. Final Answer:

   Change to WHERE salary IS NULL -> Option B

4. Quick Check:

   Use IS NULL, not = NULL [OK]

Hint: Replace = NULL with IS NULL to fix query [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Comparing NULL to zero
• Quoting NULL as string

5. You want to select all rows from orders where shipped_date is either NULL or equals '2024-01-01'. Which WHERE clause is correct?

hard

A. WHERE shipped_date IS NULL OR shipped_date = '2024-01-01'

B. WHERE shipped_date = NULL OR shipped_date = '2024-01-01'

C. WHERE shipped_date = 'NULL' OR shipped_date = '2024-01-01'

D. WHERE shipped_date != NULL OR shipped_date = '2024-01-01'

Solution

1. Step 1: Understand NULL comparison rules

   Use IS NULL to check for NULL values, not = NULL or != NULL.

2. Step 2: Analyze the WHERE clause options

   WHERE shipped_date IS NULL OR shipped_date = '2024-01-01' correctly uses IS NULL and equals comparison; others misuse NULL comparisons or treat NULL as string.

3. Final Answer:

   WHERE shipped_date IS NULL OR shipped_date = '2024-01-01' -> Option A

4. Quick Check:

   Combine IS NULL with = for dates [OK]

Hint: Use IS NULL for NULL, = for values in OR conditions [OK]

Common Mistakes:

• Using = NULL or != NULL in OR conditions
• Quoting NULL as string
• Mixing NULL checks with wrong operators