IS DISTINCT FROM for NULL-safe comparison in PostgreSQL - Mini Project: Build & Apply

Using IS DISTINCT FROM for NULL-safe Comparison in PostgreSQL

📖 Scenario: You are managing a customer database where some customers may not have provided their email addresses yet (NULL values). You want to find customers whose email addresses have changed compared to a previous record, but you need to handle NULL values safely without errors.

🎯 Goal: Build a SQL query using IS DISTINCT FROM to safely compare email addresses, including NULL values, to identify customers with changed emails.

📋 What You'll Learn

Create a table called customers with columns id (integer) and email (text).

Insert sample data with some NULL email values.

Create a table called previous_customers with the same structure and sample data.

Write a SQL query using IS DISTINCT FROM to find customers whose current email is different from the previous email, including NULL-safe comparison.

💡 Why This Matters

🌍 Real World

Handling NULL values safely is important in real databases where missing data is common. Using <code>IS DISTINCT FROM</code> helps avoid errors and incorrect results when comparing such data.

💼 Career

Database developers and analysts often need to compare records safely, especially when data can be incomplete. This skill is useful for data cleaning, auditing, and synchronization tasks.

1

Create the customers table and insert data

Create a table called customers with columns id (integer) and email (text). Insert these exact rows: (1, '[e-mail]'), (2, NULL), (3, '[e-mail]').

PostgreSQL

-- Create the customers table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add the rows exactly as specified.

2

Create the previous_customers table and insert data

Create a table called previous_customers with columns id (integer) and email (text). Insert these exact rows: (1, '[e-mail]'), (2, '[e-mail]'), (3, NULL).

PostgreSQL

CREATE TABLE customers (id INTEGER, email TEXT);
INSERT INTO customers (id, email) VALUES
  (1, '[e-mail]'),
  (2, NULL),
  (3, '[e-mail]');
-- Create the previous_customers table and insert data
-- Your code here

Hint

Repeat the table creation and insertion steps for previous_customers with the exact rows given.

3

Write a query using IS DISTINCT FROM for NULL-safe comparison

Write a SQL query that selects id and current email from customers joined with previous_customers on id, where customers.email IS DISTINCT FROM previous_customers.email.

PostgreSQL

CREATE TABLE customers (id INTEGER, email TEXT);
INSERT INTO customers (id, email) VALUES
  (1, '[e-mail]'),
  (2, NULL),
  (3, '[e-mail]');

CREATE TABLE previous_customers (id INTEGER, email TEXT);
INSERT INTO previous_customers (id, email) VALUES
  (1, '[e-mail]'),
  (2, '[e-mail]'),
  (3, NULL);

-- Write the query using IS DISTINCT FROM
-- Your code here

Hint

Use a JOIN on id and the IS DISTINCT FROM operator in the WHERE clause to compare emails safely.

4

Complete the query with ORDER BY for clear results

Add ORDER BY customers.id at the end of the query to sort the results by id.

PostgreSQL

CREATE TABLE customers (id INTEGER, email TEXT);
INSERT INTO customers (id, email) VALUES
  (1, '[e-mail]'),
  (2, NULL),
  (3, '[e-mail]');

CREATE TABLE previous_customers (id INTEGER, email TEXT);
INSERT INTO previous_customers (id, email) VALUES
  (1, '[e-mail]'),
  (2, '[e-mail]'),
  (3, NULL);

SELECT customers.id, customers.email
FROM customers
JOIN previous_customers ON customers.id = previous_customers.id
WHERE customers.email IS DISTINCT FROM previous_customers.email
-- Add ORDER BY clause here
-- Your code here

Hint

Use ORDER BY customers.id to sort the output by customer ID.

Practice

1. What does the IS DISTINCT FROM operator do in PostgreSQL when comparing two values?

easy

A. It returns true if the values are different or one is NULL and the other is not.

B. It returns true only if both values are NULL.

C. It returns false if the values are different.

D. It treats NULL as equal to any value.

Solution

1. Step 1: Understand the behavior of IS DISTINCT FROM

   This operator treats NULL as a real value and compares two values safely even if one or both are NULL.

2. Step 2: Analyze the comparison results

   It returns true if the values differ or if one is NULL and the other is not, otherwise false.

3. Final Answer:

   It returns true if the values are different or one is NULL and the other is not. -> Option A

4. Quick Check:

   IS DISTINCT FROM returns true for different or NULL vs non-NULL [OK]

Hint: Think: NULL counts as a real value in IS DISTINCT FROM [OK]

Common Mistakes:

• Assuming NULL equals NULL
• Confusing IS DISTINCT FROM with = operator
• Thinking it returns true only when both are NULL

2. Which of the following is the correct syntax to check if column a is distinct from column b in PostgreSQL?

easy

A. a <> b OR a IS NULL OR b IS NULL

B. a != b

C. a IS DISTINCT FROM b

D. a IS NOT DISTINCT FROM b

Solution

1. Step 1: Recall the correct syntax for NULL-safe comparison

   The operator IS DISTINCT FROM is used as a IS DISTINCT FROM b to compare two values safely including NULLs.

2. Step 2: Evaluate other options

   a != b uses standard inequality which fails with NULLs. a <> b OR a IS NULL OR b IS NULL is a complex expression but not the standard syntax. a IS NOT DISTINCT FROM b checks for NOT DISTINCT, which is the opposite.

3. Final Answer:

   a IS DISTINCT FROM b -> Option C

4. Quick Check:

   Use IS DISTINCT FROM for NULL-safe inequality [OK]

Hint: Use IS DISTINCT FROM for NULL-safe comparisons [OK]

Common Mistakes:

• Using != which fails with NULLs
• Writing complex OR conditions instead of operator
• Confusing IS DISTINCT FROM with IS NOT DISTINCT FROM

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

SELECT id FROM items WHERE value IS DISTINCT FROM NULL;

Assuming items has rows:
(1, NULL), (2, 10), (3, NULL), (4, 20)

medium

A. [1, 2, 3, 4]

B. [2, 4]

C. [1, 3]

D. [] (empty list)

Solution

1. Step 1: Understand the condition value IS DISTINCT FROM NULL

   This condition returns true if value is not NULL, because NULL is treated as a real value.

2. Step 2: Apply condition to each row

   Rows with value 10 and 20 are distinct from NULL, so ids 2 and 4 match.

3. Final Answer:

   [2, 4] -> Option B

4. Quick Check:

   IS DISTINCT FROM NULL filters non-NULL values [OK]

Hint: IS DISTINCT FROM NULL means NOT NULL [OK]

Common Mistakes:

• Thinking it returns rows with NULL values
• Confusing IS DISTINCT FROM with IS NULL
• Assuming it returns all rows

4. You wrote this query but it returns no rows:

SELECT * FROM users WHERE email IS DISTINCT FROM NULL;

What is the likely mistake?

medium

A. The email column has only NULL values

B. Using IS DISTINCT FROM NULL instead of IS NOT NULL

C. IS DISTINCT FROM cannot be used with NULL

D. The table users is empty

Solution

1. Step 1: Understand what email IS DISTINCT FROM NULL means

   This condition returns true for all rows where email is NOT NULL.

2. Step 2: Analyze why no rows are returned

   If no rows are returned, it means no rows satisfy the condition, so all email values are NULL or the table is empty.

3. Step 3: Eliminate other options

   Using IS DISTINCT FROM NULL instead of IS NOT NULL is incorrect because IS DISTINCT FROM NULL is valid. The table users is empty is possible but less likely. IS DISTINCT FROM cannot be used with NULL is false because the operator supports NULL.

4. Final Answer:

   The email column has only NULL values -> Option A

5. Quick Check:

   No rows means no non-NULL emails [OK]

Hint: No rows means condition never true; check NULL presence [OK]

Common Mistakes:

• Thinking IS DISTINCT FROM NULL is invalid syntax
• Assuming table is empty without checking
• Confusing IS DISTINCT FROM NULL with IS NULL

5. You want to find all pairs of rows in table records where col1 and col2 differ, treating NULLs as real values. Which query correctly finds these pairs?

hard

A. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS NOT DISTINCT FROM r2.col1 AND r1.col2 IS NOT DISTINCT FROM r2.col2;

B. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 <> r2.col1 OR r1.col2 <> r2.col2;

C. SELECT * FROM records r1 JOIN records r2 ON r1.id = r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 AND r1.col2 IS DISTINCT FROM r2.col2;

D. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 OR r1.col2 IS DISTINCT FROM r2.col2;

Solution

1. Step 1: Understand the goal

   We want pairs where col1 or col2 differ, treating NULLs as real values.

2. Step 2: Analyze each option

   SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 OR r1.col2 IS DISTINCT FROM r2.col2; uses IS DISTINCT FROM with OR, correctly finding pairs differing in either column. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 <> r2.col1 OR r1.col2 <> r2.col2; uses <> which fails with NULLs. SELECT * FROM records r1 JOIN records r2 ON r1.id = r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 AND r1.col2 IS DISTINCT FROM r2.col2; joins on equal ids, so no pairs. SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS NOT DISTINCT FROM r2.col1 AND r1.col2 IS NOT DISTINCT FROM r2.col2; finds pairs where columns are NOT distinct, opposite of goal.

3. Final Answer:

   SELECT * FROM records r1 JOIN records r2 ON r1.id <> r2.id WHERE r1.col1 IS DISTINCT FROM r2.col1 OR r1.col2 IS DISTINCT FROM r2.col2 -> Option D

4. Quick Check:

   Use IS DISTINCT FROM with OR for NULL-safe difference [OK]

Hint: Use IS DISTINCT FROM with OR to find any column differences [OK]

Common Mistakes:

• Using <> which fails with NULLs
• Joining on equal ids instead of different ids
• Using IS NOT DISTINCT FROM which finds equal rows