NULL behavior in comparisons in SQL

Introduction

NULL means we don't know the value. When we compare NULL with anything, the result is also unknown, so it behaves differently than normal values.

When checking if a database field has missing or unknown data.

When filtering rows that might have NULL values in a column.

When writing conditions that need to handle unknown or missing information.

When you want to find rows where a value is definitely missing.

When you want to avoid wrong results caused by comparing NULLs directly.

Syntax

SQL

expression IS NULL
expression IS NOT NULL

Use IS NULL or IS NOT NULL to check for NULL values because normal comparison operators (=, <>) do not work with NULL.

Comparisons like 'column = NULL' always return unknown, so they don't find NULLs.

Examples

Finds all employees who do not have a manager assigned (manager_id is unknown).

SQL

SELECT * FROM employees WHERE manager_id IS NULL;

Finds all orders that have a known delivery date.

SQL

SELECT * FROM orders WHERE delivery_date IS NOT NULL;

This returns no rows because comparing with NULL using '=' does not work.

SQL

SELECT * FROM products WHERE price = NULL;

Sample Program

This creates a table with some values and NULL. It shows how IS NULL returns true for NULL values, but '=' comparison with NULL returns unknown (NULL).

SQL

CREATE TABLE test_null (
  id INT,
  value INT
);

INSERT INTO test_null VALUES (1, 10), (2, NULL), (3, 20);

SELECT id, value, value IS NULL AS is_null, value = NULL AS equals_null FROM test_null;

Important Notes

NULL means unknown, so any comparison with NULL results in unknown, not true or false.

Always use IS NULL or IS NOT NULL to check for NULL values.

Be careful: 'value <> NULL' or 'value = NULL' will not work as expected.

Summary

NULL means missing or unknown data in databases.

Use IS NULL or IS NOT NULL to check for NULL values.

Comparisons with NULL using = or <> do not work and return unknown.

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