IS NULL vs equals NULL in SQL - Performance Comparison

Time Complexity: IS NULL vs equals NULL

O(n)

Understanding Time Complexity

We want to understand how checking for NULL values in SQL affects the time it takes to run a query.

Specifically, we ask: Does using IS NULL or = NULL change how long the query takes as data grows?

Scenario Under Consideration

Analyze the time complexity of these two queries:

-- Using IS NULL
SELECT * FROM employees WHERE manager_id IS NULL;

-- Using equals NULL
SELECT * FROM employees WHERE manager_id = NULL;
    

Both queries try to find rows where manager_id is missing, but use different syntax.

Identify Repeating Operations

Look at what repeats as the database checks each row:

• Primary operation: Checking the manager_id value for each row.
• How many times: Once per row in the employees table.

How Execution Grows With Input

As the number of rows grows, the database must check more values:

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of checks grows directly with the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the query grows in a straight line as the table gets bigger.

Common Mistake

[X] Wrong: "Using = NULL works the same as IS NULL and returns the right rows."

[OK] Correct: In SQL, = NULL never matches any row because NULL means unknown, so this condition always fails. Only IS NULL correctly finds rows with missing values.

Interview Connect

Understanding how NULL checks work and their cost helps you write correct and efficient queries, a skill valued in real projects and interviews.

Self-Check

"What if we add an index on manager_id? How would that change the time complexity of the IS NULL query?"

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