Why equals NULL fails in SQL - Performance Analysis

Time Complexity: Why equals NULL fails

O(n)

Understanding Time Complexity

When we check if a value equals NULL in SQL, the way the database processes this affects how long it takes. We want to understand how the time to run such a query changes as the data grows.

How does the database handle comparisons with NULL, and what does that mean for performance?

Scenario Under Consideration

Analyze the time complexity of the following SQL query.

SELECT *
FROM employees
WHERE department_id IS NULL;
    

This query tries to find employees whose department_id is NULL using the correct IS NULL operator.

Identify Repeating Operations

Look at what the database does repeatedly when running this query.

• Primary operation: It checks each row's department_id to see if it IS NULL.
• How many times: Once for every row in the employees table.

How Execution Grows With Input

As the number of rows grows, the database checks more rows.

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 with the number of rows.

Common Mistake

[X] Wrong: "Using '=' to compare with NULL works like normal values and returns matching rows."

[OK] Correct: In SQL, NULL means unknown, so '=' never returns true when comparing to NULL. This makes the query return no rows, even though it checks every row.

Interview Connect

Understanding how NULL comparisons work and their impact on query time helps you write correct and efficient SQL. This skill shows you know how databases handle special values and how that affects performance.

Self-Check

"What if we replace '=' with 'IS NULL' in the WHERE clause? How would the time complexity change?"

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