IS NULL vs equals NULL in SQL - Quick Revision & Key Differences

Recall & Review

beginner

What does the SQL expression IS NULL check for?

It checks if a value is NULL, meaning it has no value or is unknown.

beginner

Why does the expression column = NULL not work as expected in SQL?

Because NULL means unknown, and comparing anything to unknown with = returns UNKNOWN, not TRUE.

beginner

How do you correctly check if a column has a NULL value in SQL?

Use column IS NULL to check if the column's value is NULL.

intermediate

What is the difference between IS NULL and = NULL in SQL?

IS NULL checks if a value is NULL correctly. = NULL does not work because NULL is not a value but an unknown state.

beginner

What happens if you use WHERE column = NULL in a SQL query?

The condition will never be true, so no rows with NULL values will be returned.

Which SQL condition correctly checks if a column is NULL?

A column != NULL

B column = NULL

C column IS NULL

D column <> NULL

IS NULL is the correct way to check for NULL values in SQL.

What does NULL represent in SQL?

A An unknown or missing value

B An empty string

C A zero value

D A negative number

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

If you write WHERE column = NULL, what will happen?

A It will return all rows

B It will return no rows

C It will return rows where column is NULL

D It will cause a syntax error

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

How do you check if a column is NOT NULL in SQL?

A column != NULL

B column = NOT NULL

C column <> NULL

D column IS NOT NULL

IS NOT NULL is the correct way to check for values that are not NULL.

Why can't you use = NULL to compare values in SQL?

A Because NULL means unknown, and comparisons with unknown are not true

B Because NULL is a string

C Because NULL is a number

D Because SQL does not support NULL

NULL means unknown, so = NULL does not return true for any value.

Explain how to check for NULL values in SQL and why = NULL does not work.

Think about what NULL means in SQL and how comparisons behave.

Describe the difference between IS NULL and = NULL in SQL queries.

Focus on how SQL treats NULL in conditions.

Practice

1. Which SQL expression correctly checks if a column age has missing (NULL) values?

easy

A. age IS NULL

B. age = NULL

C. age == NULL

D. age != NULL

Solution

1. Step 1: Understand NULL in SQL

   NULL means unknown or missing data, and it cannot be compared with = or != operators.

2. Step 2: Use IS NULL to check for missing values

   The correct way to check for NULL is using IS NULL, which returns true for missing values.

3. Final Answer:

   age IS NULL -> Option A

4. Quick Check:

   Use IS NULL to check missing values [OK]

Hint: Use IS NULL, never = NULL to check missing data [OK]

Common Mistakes:

• Using = NULL returns no rows
• Confusing IS NULL with equality operator
• Using == NULL which is invalid in SQL

2. Which of the following SQL WHERE clauses is syntactically correct to find rows where salary is NULL?

easy

A. WHERE salary IS NULL

B. WHERE salary = NULL

C. WHERE salary == NULL

D. WHERE salary != NULL

Solution

1. Step 1: Review SQL syntax for NULL checks

   SQL uses IS NULL to check for NULL values; equality operators (=, ==) are invalid here.

2. Step 2: Identify the correct syntax

   Only WHERE salary IS NULL is valid and will run without error.

3. Final Answer:

   WHERE salary IS NULL -> Option A

4. Quick Check:

   IS NULL is correct syntax for NULL checks [OK]

Hint: IS NULL is the only valid syntax for NULL checks [OK]

Common Mistakes:

• Using = NULL causes syntax errors or no matches
• Using == NULL is invalid SQL
• Using != NULL does not find NULLs

3. Given the table employees with column manager_id that can be NULL, what will this query return?

SELECT * FROM employees WHERE manager_id = NULL;

medium

A. All rows where manager_id is NULL

B. Syntax error

C. No rows

D. All rows where manager_id is not NULL

Solution

1. Step 1: Understand comparison with NULL using = operator

   In SQL, = NULL does not match any rows because NULL is unknown and cannot be compared with =.

2. Step 2: Result of the query

   The query returns zero rows because no value equals NULL using = operator.

3. Final Answer:

   No rows -> Option C

4. Quick Check:

   = NULL returns no rows [OK]

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

Common Mistakes:

• Expecting = NULL to find NULL values
• Confusing NULL with empty string or zero
• Assuming syntax error instead of empty result

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

SELECT * FROM users WHERE email = NULL;

But it returns no rows. What is the fix?

medium

A. Use email IS NOT NULL

B. Change to email != NULL

C. Add quotes: email = 'NULL'

D. Change to email IS NULL

Solution

1. Step 1: Identify incorrect NULL comparison

   Using = NULL does not find NULL values because NULL is not equal to anything.

2. Step 2: Correct NULL check syntax

   Use IS NULL to properly find rows where email is missing.

3. Final Answer:

   Change to email IS NULL -> Option D

4. Quick Check:

   Use IS NULL to find missing values [OK]

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

Common Mistakes:

• Using != NULL expecting to find NULLs
• Using string 'NULL' instead of NULL keyword
• Using IS NOT NULL which finds non-missing values

5. You want to select all customers who have no recorded phone number or whose phone number is an empty string. Which WHERE clause correctly finds these rows?

SELECT * FROM customers WHERE ???

hard

A. phone = NULL OR phone = ''

B. phone IS NULL OR phone = ''

C. phone IS NULL AND phone = ''

D. phone = NULL AND phone = ''

Solution

1. Step 1: Understand NULL and empty string difference

   NULL means missing data, empty string means data is present but empty.

2. Step 2: Combine conditions correctly

   Use phone IS NULL to find missing phone numbers and phone = '' to find empty strings, combined with OR to get either case.

3. Final Answer:

   phone IS NULL OR phone = '' -> Option B

4. Quick Check:

   Use IS NULL for missing, = '' for empty [OK]

Hint: Use IS NULL OR = '' to find missing or empty values [OK]

Common Mistakes:

• Using = NULL which never matches
• Using AND instead of OR excludes all rows
• Confusing empty string with NULL