IS NULL vs equals NULL in SQL - Trade-offs & Expert Analysis

Overview - IS NULL vs equals NULL

What is it?

In SQL, NULL represents a missing or unknown value. To check if a value is NULL, you cannot use the usual equals sign (=). Instead, SQL provides a special operator called IS NULL. This topic explains the difference between using IS NULL and using equals (=) with NULL.

Why it matters

Without understanding how to properly check for NULL values, queries can return wrong or empty results. This can cause confusion and errors in data analysis, reporting, or application behavior. Knowing the correct way to test for NULL ensures accurate data handling and reliable database operations.

Where it fits

Before this, learners should understand basic SQL queries and the concept of NULL as a special marker for missing data. After this, learners can explore more advanced SQL concepts like three-valued logic, COALESCE function, and NULL handling in joins and aggregates.

Mental Model

Core Idea

NULL is not a value but a marker for unknown, so you must use IS NULL to check it, not equals (=).

Think of it like...

Imagine a box with an unknown item inside. Asking 'Is the box equal to a specific item?' makes no sense because you don't know what's inside. Instead, you ask 'Is the box empty or unknown?' which is like IS NULL.

Value check flow:

  +-------------------+
  |  Is value NULL?   |
  +---------+---------+
            |
    +-------+-------+
    |               |
  Yes             No
    |               |
Use IS NULL     Use = or !=
  operator      for comparisons

Build-Up - 7 Steps

1

Foundation

Understanding NULL in SQL

Concept: NULL means missing or unknown data, not zero or empty string.

In SQL, NULL is a special marker that means the data is missing or unknown. It is different from zero (0), empty string (''), or any other value. For example, a person's middle name might be NULL if unknown.

Result

NULL values represent unknown or missing data distinct from any actual value.

Understanding that NULL is not a value but a marker is key to handling data correctly.

2

Foundation

Why equals (=) does not work with NULL

Concept: Comparing NULL with = always returns unknown, not true or false.

When you write 'column = NULL', SQL does not treat it as true or false. Instead, it returns UNKNOWN because NULL means unknown. This means the condition fails to match any rows.

Result

Queries using '= NULL' return no rows even if NULL values exist.

Knowing that NULL breaks normal comparison logic prevents wrong query results.

3

Intermediate

Using IS NULL to check for NULL values

Concept: IS NULL is a special operator designed to test if a value is NULL.

To find rows where a column is NULL, use 'column IS NULL'. This returns true for NULL values and false otherwise. For example: SELECT * FROM table WHERE column IS NULL;

Result

The query returns all rows where the column has NULL values.

Recognizing IS NULL as the correct way to test NULL ensures accurate filtering.

4

Intermediate

Using IS NOT NULL for non-NULL values

Concept: IS NOT NULL tests if a value is not NULL, opposite of IS NULL.

To find rows where a column has any value (not NULL), use 'column IS NOT NULL'. For example: SELECT * FROM table WHERE column IS NOT NULL;

Result

The query returns all rows where the column has actual values, excluding NULLs.

Knowing IS NOT NULL helps to exclude unknown or missing data effectively.

5

Intermediate

Three-valued logic with NULL in SQL

🤔Before reading on: Do you think 'column = NULL' returns true or false? Commit to your answer.

Concept: SQL uses three-valued logic: TRUE, FALSE, and UNKNOWN when NULL is involved.

When NULL is compared with any value using '=', the result is UNKNOWN, not TRUE or FALSE. WHERE clauses only select rows where the condition is TRUE, so UNKNOWN acts like FALSE. This is why '= NULL' never matches rows.

Result

Understanding three-valued logic explains why NULL comparisons behave differently.

Understanding three-valued logic clarifies why NULL breaks usual true/false comparisons.

6

Advanced

Using COALESCE to handle NULL values

🤔Before reading on: Can you guess what COALESCE(column, default) does with NULL? Commit to your answer.

Concept: COALESCE returns the first non-NULL value from its arguments, helping handle NULLs gracefully.

COALESCE(column, default_value) returns the column's value if not NULL; otherwise, it returns default_value. This helps avoid NULL in results and simplifies queries.

Result

Queries using COALESCE replace NULLs with meaningful defaults.

Knowing COALESCE helps manage NULLs without complex IS NULL checks.

7

Expert

Why NULL comparisons can cause subtle bugs

🤔Before reading on: Do you think 'NULL = NULL' is true or false? Commit to your answer.

Concept: NULL compared to NULL with '=' is UNKNOWN, not true, which can cause unexpected query results.

Even comparing NULL to NULL using '=' returns UNKNOWN, not TRUE. To check if two values are both NULL, you must use 'IS NULL' on both or use special functions. This subtlety can cause bugs if misunderstood.

Result

Recognizing this prevents logical errors in queries involving NULLs.

Understanding that NULL = NULL is not true avoids common logical mistakes in SQL.

Under the Hood

Internally, SQL treats NULL as an unknown value without a concrete representation. Comparisons involving NULL do not return true or false but UNKNOWN, part of SQL's three-valued logic. The IS NULL operator is a special syntax that directly tests the presence of this unknown marker, bypassing normal comparison rules.

Why designed this way?

SQL's design reflects the need to represent missing or unknown data distinctly from actual values. Using three-valued logic allows queries to handle uncertainty explicitly. The IS NULL operator was introduced to provide a clear, unambiguous way to test for NULL, avoiding confusion with normal equality.

SQL NULL comparison flow:

+-------------------+
|   Value A         |
+---------+---------+
          |
          v
+-------------------+
|   Value B         |
+---------+---------+
          |
          v
+-------------------+
| Are either NULL?   |
+----+---------+----+
     |         |
    Yes       No
     |         |
+----v----+ +--v----+
| Result | | Result |
| UNKNOWN| | TRUE/  |
|        | | FALSE  |
+---------+ +-------+

IS NULL bypasses this by directly checking NULL presence.

Myth Busters - 4 Common Misconceptions

Quick: Does 'column = NULL' return rows where column is NULL? Commit yes or no.

Common Belief: Using 'column = NULL' will find rows where the column is NULL.

Reality: 'column = NULL' always returns no rows because NULL comparisons yield UNKNOWN, not TRUE.

Why it matters: Using '=' with NULL causes queries to miss NULL rows, leading to incomplete or wrong results.

Quick: Is 'NULL = NULL' true or false? Commit your answer.

Common Belief: NULL equals NULL, so 'NULL = NULL' is true.

Reality: 'NULL = NULL' is UNKNOWN, not true, so it does not behave like normal equality.

Why it matters: Assuming NULL equals NULL can cause logic errors when checking for missing data equality.

Quick: Does 'IS NULL' work like '= NULL'? Commit yes or no.

Common Belief: 'IS NULL' is just a different way to write '= NULL'.

Reality: 'IS NULL' is a special operator that correctly tests for NULL, unlike '= NULL' which fails.

Why it matters: Confusing these leads to wrong queries and data filtering mistakes.

Quick: Can you use '!=' to find non-NULL values? Commit yes or no.

Common Belief: Using 'column != NULL' finds rows where column is not NULL.

Reality: 'column != NULL' returns UNKNOWN for NULLs and does not reliably find non-NULL rows; use 'IS NOT NULL' instead.

Why it matters: Using '!=' with NULL causes unexpected missing rows in results.

Expert Zone

1

NULL handling varies slightly between SQL dialects, affecting query portability.

2

Indexes may not include NULL values depending on database settings, impacting performance.

3

Three-valued logic affects not only WHERE clauses but also JOIN conditions and CHECK constraints.

When NOT to use

Avoid using '=' or '!=' to test for NULL; always use IS NULL or IS NOT NULL. For replacing NULLs in output, use COALESCE or CASE expressions. When precise NULL semantics are not needed, consider default values to simplify logic.

Production Patterns

In production, IS NULL checks are common in data validation, filtering, and conditional logic. COALESCE is widely used to provide default values in reports and user interfaces. Understanding NULL behavior is critical for writing reliable JOINs and aggregate queries.

Connections

Three-valued logic

IS NULL and NULL comparisons are direct applications of three-valued logic in SQL.

Knowing three-valued logic explains why NULL comparisons behave differently from normal boolean logic.

Data cleaning

Handling NULLs properly is essential in data cleaning to avoid misinterpretation of missing data.

Understanding IS NULL helps in identifying and managing missing data during cleaning processes.

Philosophy of knowledge

The concept of NULL as unknown parallels philosophical ideas about unknown or undefined knowledge.

Recognizing NULL as unknown rather than a value connects database logic to broader ideas about uncertainty.

Common Pitfalls

#1 Using '=' to check for NULL values returns no results.

Wrong approach:

SELECT * FROM users WHERE middle_name = NULL;

Correct approach:

SELECT * FROM users WHERE middle_name IS NULL;

Root cause:

Misunderstanding that NULL is not a value and '=' does not work with NULL.

#2 Using '!=' to find non-NULL values misses rows with NULL.

Wrong approach:

SELECT * FROM orders WHERE delivery_date != NULL;

Correct approach:

SELECT * FROM orders WHERE delivery_date IS NOT NULL;

Root cause:

Assuming '!=' works like 'IS NOT NULL' for NULL values.

#3 Assuming 'NULL = NULL' is true in conditions.

Wrong approach:

SELECT * FROM products WHERE discount = NULL;

Correct approach:

SELECT * FROM products WHERE discount IS NULL;

Root cause:

Not realizing NULL comparisons yield UNKNOWN, not TRUE.

Key Takeaways

NULL in SQL means unknown or missing data, not a regular value.

You must use IS NULL or IS NOT NULL to check for NULL values, not '=' or '!='.

Comparisons with NULL return UNKNOWN, which behaves differently from TRUE or FALSE.

Functions like COALESCE help handle NULLs by providing default values.

Understanding NULL behavior is essential to write correct and reliable SQL queries.

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