WHERE with NOT operator in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of NOT operator in WHERE clause

Given a table Employees with columns id, name, and department, what is the output of the following query?

SELECT name FROM Employees WHERE NOT department = 'Sales';

SQL

CREATE TABLE Employees (id INT, name VARCHAR(50), department VARCHAR(50));
INSERT INTO Employees VALUES (1, 'Alice', 'Sales'), (2, 'Bob', 'HR'), (3, 'Charlie', 'Sales'), (4, 'Diana', 'IT');

A No rows returned

B Alice, Charlie

C Alice, Bob, Charlie, Diana

D Bob, Diana

💡 Hint

The NOT operator reverses the condition inside the WHERE clause.

The query selects employees whose department is NOT 'Sales'. Bob and Diana belong to 'HR' and 'IT' respectively, so they are returned.

🧠 Conceptual

intermediate

Understanding NOT with multiple conditions

Consider the query:

SELECT * FROM Products WHERE NOT (category = 'Electronics' OR price > 1000);

Which rows will this query return?

A Rows where category is 'Electronics' or price is more than 1000

B Rows where category is 'Electronics' and price is more than 1000

C Rows where category is not 'Electronics' and price is 1000 or less

D All rows in the table

💡 Hint

NOT reverses the entire condition inside the parentheses.

The condition inside parentheses is true if category is 'Electronics' or price is greater than 1000. NOT reverses this, so only rows where both are false are returned.

📝 Syntax

advanced

Identify the syntax error with NOT operator

Which of the following SQL queries will cause a syntax error?

A SELECT * FROM Orders WHERE NOT (status = 'Shipped' AND);

B SELECT * FROM Orders WHERE NOT(status = 'Shipped');

C SELECT * FROM Orders WHERE NOT status = 'Shipped' AND date > '2023-01-01';

D SELECT * FROM Orders WHERE NOT status = 'Shipped';

💡 Hint

Look for incomplete conditions inside parentheses.

Option A has an incomplete condition inside the parentheses after AND, causing a syntax error.

❓ optimization

advanced

Optimizing NOT conditions in WHERE clause

Which query is more efficient for filtering rows where status is not 'Active'?

A SELECT * FROM Users WHERE NOT status = 'Active';

B SELECT * FROM Users WHERE status <> 'Active';

C SELECT * FROM Users WHERE status != 'Active';

D SELECT * FROM Users WHERE NOT (status = 'Active');

💡 Hint

Consider how the database engine optimizes comparison operators.

Using the inequality operator (<>) is generally more efficient and clearer than using NOT with equality.

🔧 Debug

expert

Debugging unexpected results with NOT operator

A developer wrote this query:

SELECT * FROM Customers WHERE NOT city = 'New York' OR country = 'USA';

They expected to get customers who are not from New York and are from the USA. Why does this query return more rows than expected?

A Because OR has lower precedence than NOT, so the condition is evaluated as (NOT city = 'New York') OR country = 'USA', returning rows where city is not New York or country is USA

B Because NOT applies only to city, so it excludes only New York customers

C Because the query syntax is invalid and returns all rows

D Because the query should use AND instead of OR to combine conditions

💡 Hint

Think about operator precedence and how NOT and OR combine.

NOT has higher precedence than OR, so the condition is evaluated as (NOT city = 'New York') OR country = 'USA'. This returns rows where city is not New York or country is USA, which is more than expected.

Practice

1. What does the NOT operator do in a SQL WHERE clause?

easy

A. It sorts the rows in descending order.

B. It includes only rows that match the condition.

C. It excludes rows that match the condition.

D. It groups rows by the condition.

Solution

1. Step 1: Understand the role of WHERE clause

   The WHERE clause filters rows based on a condition.

2. Step 2: Understand the effect of NOT operator

   The NOT operator reverses the condition, excluding rows that match it.

3. Final Answer:

   It excludes rows that match the condition. -> Option C

4. Quick Check:

   NOT excludes matching rows = A [OK]

Hint: NOT reverses condition to exclude matching rows [OK]

Common Mistakes:

• Thinking NOT includes matching rows
• Confusing NOT with sorting or grouping
• Using NOT outside WHERE clause incorrectly

2. Which of the following is the correct syntax using the NOT operator to select rows where the column status is NOT equal to 'active'?

easy

A. SELECT * FROM users WHERE status = NOT 'active';

B. SELECT * FROM users WHERE status NOT = 'active';

C. SELECT * FROM users WHERE status <> 'active';

D. SELECT * FROM users WHERE NOT (status = 'active');

Solution

1. Step 1: Check syntax for NOT operator usage

   The NOT operator must be placed before a condition enclosed in parentheses for clarity.

2. Step 2: Evaluate each option's syntax correctness

   SELECT * FROM users WHERE status = NOT 'active'; is invalid syntax (NOT expects boolean); SELECT * FROM users WHERE status NOT = 'active'; is invalid syntax; SELECT * FROM users WHERE status <> 'active'; uses <> which means NOT equal but no NOT operator; SELECT * FROM users WHERE NOT (status = 'active'); correctly uses NOT with parentheses.

3. Final Answer:

   SELECT * FROM users WHERE NOT (status = 'active'); -> Option D

4. Quick Check:

   Correct NOT syntax uses parentheses = D [OK]

Hint: Use NOT before condition with parentheses for clarity [OK]

Common Mistakes:

• Writing NOT after column name
• Omitting parentheses around condition
• Confusing <> with NOT operator

3. Given the table employees with a column department, what will this query return?

SELECT name FROM employees WHERE NOT department = 'Sales';

medium

A. All employees not in the Sales department.

B. All employees in the Sales department.

C. Only employees with department NULL.

D. Syntax error due to NOT usage.

Solution

1. Step 1: Understand the condition NOT department = 'Sales'

   This means selecting rows where department is NOT 'Sales'.

2. Step 2: Determine which employees match this condition

   Employees in any department except 'Sales' will be returned.

3. Final Answer:

   All employees not in the Sales department. -> Option A

4. Quick Check:

   NOT department = 'Sales' excludes Sales = B [OK]

Hint: NOT reverses condition to exclude matching rows [OK]

Common Mistakes:

• Thinking it returns only Sales employees
• Assuming NULL departments are included automatically
• Believing NOT causes syntax error

4. Identify the error in this query:

SELECT * FROM orders WHERE NOT order_date > '2023-01-01';

medium

A. The query is correct and will run without errors.

B. The NOT operator cannot be used with comparison operators.

C. Parentheses are needed around the condition after NOT.

D. The date format is invalid.

Solution

1. Step 1: Check if NOT can be used with comparison operators

   NOT can be applied to any condition, including comparisons.

2. Step 2: Verify syntax and date format

   The syntax is valid and the date format 'YYYY-MM-DD' is standard in SQL.

3. Final Answer:

   The query is correct and will run without errors. -> Option A

4. Quick Check:

   NOT works with comparisons; syntax is valid = A [OK]

Hint: NOT works with any condition, parentheses optional but clearer [OK]

Common Mistakes:

• Thinking NOT needs parentheses always
• Assuming NOT can't negate > operator
• Believing date format is wrong

5. You want to find customers who have NOT placed any orders in 2023. Which query correctly uses NOT to achieve this?

Customers table: customer_id, name
Orders table: order_id, customer_id, order_date

Assume the orders.customer_id column may contain NULL values.

hard

A. SELECT name FROM Customers WHERE NOT customer_id IN (SELECT customer_id FROM Orders WHERE order_date >= '2023-01-01' AND order_date < '2024-01-01');

B. SELECT name FROM Customers WHERE NOT EXISTS (SELECT * FROM Orders WHERE Orders.customer_id = Customers.customer_id AND order_date >= '2023-01-01' AND order_date < '2024-01-01');

C. SELECT name FROM Customers WHERE customer_id NOT IN (SELECT customer_id FROM Orders WHERE order_date >= '2023-01-01' AND order_date < '2024-01-01');

D. SELECT name FROM Customers WHERE EXISTS (SELECT * FROM Orders WHERE Orders.customer_id = Customers.customer_id AND NOT order_date >= '2023-01-01' AND order_date < '2024-01-01');

Solution

1. Step 1: Understand the goal

   We want customers with no orders in 2023.

2. Step 2: Analyze each query's logic

   SELECT name FROM Customers WHERE NOT customer_id IN (SELECT customer_id FROM Orders WHERE order_date >= '2023-01-01' AND order_date < '2024-01-01'); incorrectly negates the whole IN clause with NOT outside, which is valid but less clear. SELECT name FROM Customers WHERE customer_id NOT IN (SELECT customer_id FROM Orders WHERE order_date >= '2023-01-01' AND order_date < '2024-01-01'); is correct SQL syntax but less safe if NULLs exist. SELECT name FROM Customers WHERE NOT EXISTS (SELECT * FROM Orders WHERE Orders.customer_id = Customers.customer_id AND order_date >= '2023-01-01' AND order_date < '2024-01-01'); uses NOT EXISTS with a subquery checking orders in 2023, which is the safest and most accurate method. SELECT name FROM Customers WHERE EXISTS (SELECT * FROM Orders WHERE Orders.customer_id = Customers.customer_id AND NOT order_date >= '2023-01-01' AND order_date < '2024-01-01'); uses EXISTS with NOT on order_date condition, which will not correctly exclude customers with orders in 2023.

3. Final Answer:

   SELECT name FROM Customers WHERE NOT EXISTS (SELECT * FROM Orders WHERE Orders.customer_id = Customers.customer_id AND order_date >= '2023-01-01' AND order_date < '2024-01-01'); -> Option B

4. Quick Check:

   NOT EXISTS correctly finds customers without 2023 orders = C [OK]

Hint: Use NOT EXISTS for safe exclusion of related rows [OK]

Common Mistakes:

• Using NOT IN which fails with NULLs
• Misplacing NOT inside EXISTS condition
• Confusing EXISTS and NOT EXISTS logic