NULLIF function in MySQL - Mini Project: Build & Apply

Using the NULLIF Function in MySQL

📖 Scenario: You are managing a small online store database. You want to handle cases where the discount price is the same as the original price by showing NULL instead of the discount price.

🎯 Goal: Build a MySQL query that uses the NULLIF function to return NULL when the discount price equals the original price, otherwise return the discount price.

📋 What You'll Learn

Create a table called products with columns id, name, price, and discount_price

Insert three products with specific prices and discount prices

Write a SELECT query using NULLIF(discount_price, price) to show NULL when discount price equals price

Complete the query to select id, name, price, and the discount_price or NULL using NULLIF

💡 Why This Matters

🌍 Real World

Online stores often need to show discounts only when they differ from the original price. NULLIF helps handle this cleanly in queries.

💼 Career

Understanding NULLIF is useful for database developers and analysts to write clear queries that handle special cases like equal values.

1

Create the products table

Create a table called products with columns id as INT primary key, name as VARCHAR(50), price as DECIMAL(10,2), and discount_price as DECIMAL(10,2).

MySQL

-- Create the products table with the specified columns
-- Your code here

Hint

Use CREATE TABLE with the exact column names and types.

2

Insert product data

Insert three rows into products with these exact values: (1, 'Laptop', 1000.00, 900.00), (2, 'Mouse', 25.00, 25.00), and (3, 'Keyboard', 45.00, 40.00).

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  discount_price DECIMAL(10,2)
);
-- Insert the three products with exact values below
-- Your code here

Hint

Use a single INSERT INTO statement with all three rows.

3

Write SELECT query using NULLIF

Write a SELECT query to get id, name, price, and use NULLIF(discount_price, price) as discount_price from the products table.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  discount_price DECIMAL(10,2)
);

INSERT INTO products (id, name, price, discount_price) VALUES
(1, 'Laptop', 1000.00, 900.00),
(2, 'Mouse', 25.00, 25.00),
(3, 'Keyboard', 45.00, 40.00);

-- Write SELECT query using NULLIF below
-- Your code here

Hint

Use NULLIF(discount_price, price) in the SELECT list with alias discount_price.

4

Complete the query with ordering

Add an ORDER BY id clause at the end of the SELECT query to sort the results by id.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  discount_price DECIMAL(10,2)
);

INSERT INTO products (id, name, price, discount_price) VALUES
(1, 'Laptop', 1000.00, 900.00),
(2, 'Mouse', 25.00, 25.00),
(3, 'Keyboard', 45.00, 40.00);

SELECT id, name, price, NULLIF(discount_price, price) AS discount_price FROM products
-- Add ORDER BY id below
-- Your code here

Hint

Use ORDER BY id at the end of the query to sort results.

Practice

1. What does the NULLIF function do in MySQL?

easy

A. Returns the sum of two expressions

B. Returns NULL if two expressions are equal, otherwise returns the first expression

C. Returns the first non-NULL expression

D. Returns the second expression if it is NULL

Solution

1. Step 1: Understand the purpose of NULLIF

   NULLIF compares two expressions and returns NULL if they are equal.

2. Step 2: Identify the return value when expressions differ

   If the expressions are not equal, NULLIF returns the first expression.

3. Final Answer:

   Returns NULL if two expressions are equal, otherwise returns the first expression -> Option B

4. Quick Check:

   NULLIF returns NULL on equality [OK]

Hint: NULLIF returns NULL only when values match exactly [OK]

Common Mistakes:

• Confusing NULLIF with COALESCE
• Thinking it sums values
• Assuming it returns second expression

2. Which of the following is the correct syntax for the NULLIF function in MySQL?

easy

A. NULLIF(expression1 OR expression2)

B. NULLIF(expression1 = expression2)

C. NULLIF(expression1, expression2, expression3)

D. NULLIF(expression1, expression2)

Solution

1. Step 1: Recall the syntax format of NULLIF

   NULLIF takes exactly two expressions separated by a comma inside parentheses.

2. Step 2: Check each option's syntax

   NULLIF(expression1, expression2) matches the correct syntax: NULLIF(expression1, expression2). Others have wrong number of arguments or invalid operators.

3. Final Answer:

   NULLIF(expression1, expression2) -> Option D

4. Quick Check:

   NULLIF syntax uses two expressions separated by comma [OK]

Hint: NULLIF always uses two expressions separated by a comma [OK]

Common Mistakes:

• Using comparison operator inside NULLIF
• Providing more than two arguments
• Using logical operators inside NULLIF

3. Given the table sales with columns id and amount, what will this query return?

SELECT id, NULLIF(amount, 0) AS adjusted_amount FROM sales;

If the table data is:
id | amount
1 | 100
2 | 0
3 | 50

medium

A. [{id:1, adjusted_amount:100}, {id:2, adjusted_amount:0}, {id:3, adjusted_amount:50}]

B. [{id:1, adjusted_amount:NULL}, {id:2, adjusted_amount:0}, {id:3, adjusted_amount:NULL}]

C. [{id:1, adjusted_amount:100}, {id:2, adjusted_amount:NULL}, {id:3, adjusted_amount:50}]

D. [{id:1, adjusted_amount:NULL}, {id:2, adjusted_amount:NULL}, {id:3, adjusted_amount:NULL}]

Solution

1. Step 1: Understand NULLIF(amount, 0)

   NULLIF returns NULL if amount equals 0, else returns amount.

2. Step 2: Apply to each row

   Row 1: amount=100, not 0, returns 100.
   Row 2: amount=0, equals 0, returns NULL.
   Row 3: amount=50, not 0, returns 50.

3. Final Answer:

   [{id:1, adjusted_amount:100}, {id:2, adjusted_amount:NULL}, {id:3, adjusted_amount:50}] -> Option C

4. Quick Check:

   NULLIF returns NULL only when amount is zero [OK]

Hint: NULLIF returns NULL when first value equals second [OK]

Common Mistakes:

• Assuming NULLIF returns 0 instead of NULL
• Confusing order of arguments
• Thinking NULLIF changes non-zero values

4. Identify the error in this query:

SELECT id, NULLIF(amount) FROM sales;

medium

A. NULLIF requires exactly two arguments

B. NULLIF cannot be used in SELECT clause

C. amount column does not exist

D. NULLIF should be written as NULLIF(amount = 0)

Solution

1. Step 1: Check NULLIF function usage

   NULLIF requires two arguments: expression1 and expression2.

2. Step 2: Analyze the query

   The query uses NULLIF with only one argument, which is invalid syntax.

3. Final Answer:

   NULLIF requires exactly two arguments -> Option A

4. Quick Check:

   NULLIF needs two expressions separated by comma [OK]

Hint: NULLIF always needs two arguments [OK]

Common Mistakes:

• Using NULLIF with one argument
• Writing NULLIF with comparison inside
• Assuming NULLIF works like ISNULL

5. You want to replace zero values in the score column with NULL in a query result. Which query correctly uses NULLIF to achieve this?

hard

A. SELECT id, NULLIF(score, 0) AS score_or_null FROM results;

B. SELECT id, NULLIF(0, score) AS score_or_null FROM results;

C. SELECT id, NULLIF(score) AS score_or_null FROM results;

D. SELECT id, NULLIF(score = 0) AS score_or_null FROM results;

Solution

1. Step 1: Understand the goal

   We want to convert zero scores to NULL, keep others unchanged.

2. Step 2: Apply NULLIF correctly

   NULLIF(score, 0) returns NULL if score is 0, else returns score.
   SELECT id, NULLIF(score, 0) AS score_or_null FROM results; matches this logic.

3. Step 3: Check other options

   SELECT id, NULLIF(0, score) AS score_or_null FROM results; reverses arguments, changing logic.
   SELECT id, NULLIF(score) AS score_or_null FROM results; has missing argument.
   SELECT id, NULLIF(score = 0) AS score_or_null FROM results; uses invalid syntax.

4. Final Answer:

   SELECT id, NULLIF(score, 0) AS score_or_null FROM results; -> Option A

5. Quick Check:

   NULLIF(score, 0) replaces zeros with NULL [OK]

Hint: Use NULLIF(column, 0) to convert zeros to NULL [OK]

Common Mistakes:

• Swapping argument order
• Using only one argument
• Using comparison inside NULLIF