NULL behavior in comparisons in SQL - Step-by-Step Execution

Concept Flow - NULL behavior in comparisons

Start Comparison

↓

Is any operand NULL?

No→Evaluate comparison normally

Yes↓

Result is UNKNOWN (treated as FALSE in WHERE)

↓

Use IS NULL or IS NOT NULL to check NULL explicitly

When comparing values, if any side is NULL, the result is UNKNOWN, not TRUE or FALSE. Use IS NULL or IS NOT NULL to check for NULL explicitly.

Execution Sample

SQL

SELECT * FROM users WHERE age = NULL;
SELECT * FROM users WHERE age IS NULL;

Shows that 'age = NULL' returns no rows, but 'age IS NULL' returns rows where age is NULL.

Execution Table

Step	Expression	Operands	Evaluation	Result
1	age = 25	age=25, 25	25 = 25	TRUE
2	age = NULL	age=25, NULL	25 = NULL	UNKNOWN (treated as FALSE)
3	age = NULL	age=NULL, NULL	NULL = NULL	UNKNOWN (treated as FALSE)
4	age IS NULL	age=NULL	Check if age is NULL	TRUE
5	age IS NULL	age=25	Check if age is NULL	FALSE
6	age <> NULL	age=25, NULL	25 <> NULL	UNKNOWN (treated as FALSE)
7	age IS NOT NULL	age=25	Check if age is NOT NULL	TRUE
8	age IS NOT NULL	age=NULL	Check if age is NOT NULL	FALSE

💡 Comparisons with NULL result in UNKNOWN, which acts like FALSE in WHERE filters; use IS NULL or IS NOT NULL for explicit NULL checks.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8
age	25 or NULL	25 or NULL	25 or NULL	25 or NULL	25 or NULL	25 or NULL	25 or NULL	25 or NULL
comparison_result	N/A	UNKNOWN	UNKNOWN	TRUE or FALSE	TRUE or FALSE	UNKNOWN	TRUE or FALSE	TRUE or FALSE

Key Moments - 3 Insights

Why does 'age = NULL' not return rows even if some ages are NULL?

Because any comparison with NULL results in UNKNOWN, which is treated as FALSE in WHERE clauses. See execution_table rows 2 and 3.

How can I correctly check if a column is NULL?

Use 'IS NULL' or 'IS NOT NULL' operators, which explicitly test for NULL values. See execution_table rows 4 and 5.

Is NULL equal to NULL in SQL?

No, 'NULL = NULL' results in UNKNOWN, not TRUE. Use 'IS NULL' to check for NULL. See execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of 'age = NULL' when age is 25?

A TRUE

B FALSE

C UNKNOWN

D NULL

💡 Hint

Check row 2 in the execution_table where age=25 and comparison is 'age = NULL'.

At which step does the expression correctly identify NULL values?

A Step 2

B Step 4

C Step 6

D Step 7

💡 Hint

Look for 'IS NULL' usage in the execution_table.

If you replace 'age = NULL' with 'age IS NULL', how does the result change?

A From UNKNOWN to TRUE

B From TRUE to FALSE

C From UNKNOWN to TRUE or FALSE

D No change

💡 Hint

Compare rows 2, 3 with rows 4, 5 in the execution_table.

Concept Snapshot

NULL in SQL means unknown value.
Comparisons with NULL return UNKNOWN, not TRUE or FALSE.
WHERE treats UNKNOWN as FALSE, so no rows match.
Use IS NULL or IS NOT NULL to check for NULL explicitly.
NULL = NULL is UNKNOWN, not TRUE.

Full Transcript

In SQL, NULL represents an unknown or missing value. When you compare any value to NULL using standard operators like '=', '<>', or '<', the result is not TRUE or FALSE but UNKNOWN. This UNKNOWN behaves like FALSE in WHERE clauses, so no rows match such conditions. To properly check if a column contains NULL, you must use the IS NULL or IS NOT NULL operators. For example, 'age = NULL' returns no rows even if some ages are NULL, but 'age IS NULL' returns those rows. Also, NULL is not equal to NULL; 'NULL = NULL' is UNKNOWN. Remember to use IS NULL for explicit NULL checks.

Practice

1. What does NULL represent in a database?

easy

A. False boolean

B. Zero value

C. Empty string

D. Missing or unknown data

Solution

1. Step 1: Understand the meaning of NULL

   NULL in databases means the data is missing or unknown, not zero or empty.

2. Step 2: Differentiate NULL from other values

   Zero, empty string, and false are actual values, while NULL means no value is stored.

3. Final Answer:

   Missing or unknown data -> Option D

4. Quick Check:

   NULL means missing or unknown data [OK]

Hint: NULL means unknown or missing, not zero or empty [OK]

Common Mistakes:

• Confusing NULL with zero
• Thinking NULL is an empty string
• Assuming NULL means false

2. Which of the following is the correct way to check if a column col is NULL in SQL?

easy

A. WHERE col = NULL

B. WHERE col == NULL

C. WHERE col IS NULL

D. WHERE col <> NULL

Solution

1. Step 1: Recall SQL syntax for NULL checks

   In SQL, to check for NULL, you must use IS NULL because = NULL does not work.

2. Step 2: Eliminate incorrect syntax

   col = NULL and col == NULL are invalid for NULL checks; col <> NULL also does not work.

3. Final Answer:

   WHERE col IS NULL -> Option C

4. Quick Check:

   Use IS NULL to check NULL values [OK]

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

Common Mistakes:

• Using = NULL instead of IS NULL
• Using == NULL which is invalid in SQL
• Using <> NULL to check for NULL

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

SELECT * FROM users WHERE age <> 30;

medium

A. All rows where age is not 30, including rows with NULL age

B. Only rows where age is not 30 and not NULL

C. Only rows where age is 30

D. No rows, because NULL causes error

Solution

1. Step 1: Understand comparison with NULL

   Comparisons like age <> 30 return UNKNOWN for NULL values, so those rows are excluded.

2. Step 2: Determine which rows are returned

   Only rows where age is not 30 and age is not NULL will be returned.

3. Final Answer:

   Only rows where age is not 30 and not NULL -> Option B

4. Quick Check:

   Comparisons with NULL exclude NULL rows [OK]

Hint: Comparisons exclude NULL rows, so NULLs are ignored [OK]

Common Mistakes:

• Assuming NULL rows are included
• Thinking NULL causes errors in comparisons
• Confusing <> with IS NOT NULL

4. You wrote this query to find rows where status is not NULL:

SELECT * FROM orders WHERE status <> NULL;

Why does it return no rows, and how to fix it?

medium

A. Comparison with NULL returns UNKNOWN; use status IS NOT NULL

B. Syntax error; use status != NULL

C. NULL is treated as zero; use status <> 0

D. Query is correct; no rows have status

Solution

1. Step 1: Understand NULL comparison behavior

   Comparing with NULL using <> returns UNKNOWN, so no rows match.

2. Step 2: Use correct syntax to check NOT NULL

   Use status IS NOT NULL to find rows where status is not NULL.

3. Final Answer:

   Comparison with NULL returns UNKNOWN; use status IS NOT NULL -> Option A

4. Quick Check:

   Use IS NOT NULL to check for non-NULL values [OK]

Hint: Use IS NOT NULL, not <> NULL, to find non-NULL rows [OK]

Common Mistakes:

• Using <> NULL instead of IS NOT NULL
• Assuming <> NULL works like != NULL
• Thinking NULL is zero or empty

5. You want to select all rows from products where discount is either NULL or greater than 10. Which query correctly returns these rows?

hard

A. SELECT * FROM products WHERE discount > 10 OR discount IS NULL;

B. SELECT * FROM products WHERE discount > 10 AND discount IS NULL;

C. SELECT * FROM products WHERE discount > 10 OR discount = NULL;

D. SELECT * FROM products WHERE discount > 10 AND discount = NULL;

Solution

1. Step 1: Understand NULL checks with OR condition

   To include NULL values, use IS NULL, not = NULL.

2. Step 2: Combine conditions correctly

   Use OR to select rows where discount is greater than 10 or discount is NULL.

3. Final Answer:

   SELECT * FROM products WHERE discount > 10 OR discount IS NULL; -> Option A

4. Quick Check:

   Use IS NULL with OR to include NULLs [OK]

Hint: Use OR with IS NULL to include NULL values in conditions [OK]

Common Mistakes:

• Using = NULL instead of IS NULL
• Using AND instead of OR with NULL checks
• Assuming NULL compares like normal values