IS NULL vs equals NULL in SQL - Hands-On Comparison

Understanding IS NULL vs equals NULL in SQL

📖 Scenario: You are managing a small library database. Some books have a known publication year, but some do not have this information yet, so their publication year is stored as NULL.

🎯 Goal: You will create a table with book data including some NULL values, then write queries to find books with and without publication years using IS NULL and = NULL.

📋 What You'll Learn

Create a table called books with columns id, title, and pub_year

Insert 4 books with exact titles and publication years, including NULL for unknown years

Write a query using IS NULL to find books without a publication year

Write a query using = NULL to find books without a publication year (to see it returns no rows)

Write a query using IS NOT NULL to find books with a known publication year

💡 Why This Matters

🌍 Real World

Handling NULL values is common in databases when some information is missing or unknown, like missing publication years in a library catalog.

💼 Career

Understanding how to query NULL values correctly is essential for data analysis, reporting, and database management jobs.

1

Create the books table and insert data

Create a table called books with columns id (integer), title (text), and pub_year (integer). Then insert these exact rows: (1, 'The Hobbit', 1937), (2, '1984', 1949), (3, 'Unknown Book', NULL), (4, 'Future Book', NULL).

SQL

-- Create table books and insert 4 rows with some NULL pub_year values
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows. Use NULL for unknown publication years.

2

Write a query using IS NULL to find books without a publication year

Write a SQL query that selects all columns from books where pub_year IS NULL to find books without a publication year.

SQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  pub_year INTEGER
);

INSERT INTO books (id, title, pub_year) VALUES
  (1, 'The Hobbit', 1937),
  (2, '1984', 1949),
  (3, 'Unknown Book', NULL),
  (4, 'Future Book', NULL);

-- Write a query to select all books where pub_year IS NULL
-- Your code here

Hint

Use IS NULL to check for NULL values in SQL.

3

Write a query using = NULL to find books without a publication year

Write a SQL query that selects all columns from books where pub_year = NULL. This will show that using = NULL does not work as expected.

SQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  pub_year INTEGER
);

INSERT INTO books (id, title, pub_year) VALUES
  (1, 'The Hobbit', 1937),
  (2, '1984', 1949),
  (3, 'Unknown Book', NULL),
  (4, 'Future Book', NULL);

SELECT * FROM books WHERE pub_year IS NULL;

-- Write a query to select all books where pub_year = NULL
-- Your code here

Hint

Try using = NULL in the WHERE clause and observe that it returns no rows.

4

Write a query using IS NOT NULL to find books with a known publication year

Write a SQL query that selects all columns from books where pub_year IS NOT NULL to find books that have a known publication year.

SQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  pub_year INTEGER
);

INSERT INTO books (id, title, pub_year) VALUES
  (1, 'The Hobbit', 1937),
  (2, '1984', 1949),
  (3, 'Unknown Book', NULL),
  (4, 'Future Book', NULL);

SELECT * FROM books WHERE pub_year IS NULL;

SELECT * FROM books WHERE pub_year = NULL;

-- Write a query to select all books where pub_year IS NOT NULL
-- Your code here

Hint

Use IS NOT NULL to find rows where the column has a value.

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