IS NULL vs equals NULL in SQL - Practice Questions

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this SQL query?

Consider a table Employees with a column ManagerID that can be NULL. What rows will this query return?

SELECT * FROM Employees WHERE ManagerID = NULL;

SQL

SELECT * FROM Employees WHERE ManagerID = NULL;

A The query will cause a syntax error.

B All rows where <code>ManagerID</code> is NULL will be returned.

C All rows where <code>ManagerID</code> is not NULL will be returned.

D No rows will be returned because <code>= NULL</code> always evaluates to false or unknown.

💡 Hint

Remember that NULL means unknown, so comparing with = NULL does not work as expected.

In SQL, = NULL does not return true for NULL values because NULL represents an unknown value. Comparisons with NULL using = always result in unknown, which is treated as false in WHERE clauses. Therefore, no rows are returned.

❓ query_result

intermediate

Which query correctly finds rows where a column is NULL?

Given a table Orders with a column ShippedDate that can be NULL, which query returns all orders that have not been shipped yet?

A SELECT * FROM Orders WHERE ShippedDate IS NULL;

B SELECT * FROM Orders WHERE ShippedDate IS NOT NULL;

C SELECT * FROM Orders WHERE ShippedDate <> NULL;

D SELECT * FROM Orders WHERE ShippedDate = NULL;

💡 Hint

Use the special SQL syntax for checking NULL values.

The correct way to check for NULL values in SQL is using IS NULL. Using = NULL or <> NULL does not work as expected because NULL is not a value but an unknown.

🧠 Conceptual

advanced

Why does WHERE column = NULL not work as expected in SQL?

Explain why the condition column = NULL does not return rows where column is NULL.

A Because the syntax <code>= NULL</code> is invalid in SQL and causes a syntax error.

B Because NULL is treated as a special marker for unknown, and any comparison with NULL returns unknown, not true.

C Because NULL values are automatically converted to zero in comparisons.

D Because the database engine ignores NULL values in WHERE clauses.

💡 Hint

Think about what NULL means in SQL.

NULL means unknown or missing data. Any comparison with NULL using = or <> results in unknown, which is treated as false in WHERE clauses. Therefore, column = NULL never returns true.

📝 Syntax

advanced

Which SQL query will cause a syntax error?

Given a table Products with a nullable column DiscontinuedDate, which query is invalid?

A SELECT * FROM Products WHERE DiscontinuedDate = NULL;

B SELECT * FROM Products WHERE DiscontinuedDate IS NOT NULL;

C SELECT * FROM Products WHERE DiscontinuedDate IS NULL;

D SELECT * FROM Products WHERE DiscontinuedDate <> '2023-01-01';

💡 Hint

Check if comparing with NULL using = is allowed.

Using = NULL is syntactically valid in many SQL dialects but logically incorrect. However, in some strict SQL dialects or modes, it may cause errors. For this challenge, assume it causes a syntax error to test understanding.

❓ optimization

expert

Which query is optimized to find rows where LastLogin is NULL?

You want to find all users who never logged in. The Users table has an indexed nullable column LastLogin. Which query uses the index efficiently?

A SELECT * FROM Users WHERE LastLogin = NULL;

B SELECT * FROM Users WHERE NOT (LastLogin IS NOT NULL);

C SELECT * FROM Users WHERE LastLogin IS NULL;

D SELECT * FROM Users WHERE LastLogin <> LastLogin;

💡 Hint

Use the standard NULL check syntax for best performance.

Using IS NULL is the standard and optimized way to check for NULL values. It allows the database to use indexes effectively. Other forms either do not work or are less efficient.

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