NULL in DISTINCT, GROUP BY, and ORDER BY in SQL

Introduction

We use NULL to represent missing or unknown values in data. Understanding how NULL behaves with DISTINCT, GROUP BY, and ORDER BY helps us organize and summarize data correctly.

When you want to find unique values in a column that may have missing data.

When grouping data to summarize, and some groups might have NULL values.

When sorting data that includes NULLs, to know where they appear in the order.

Syntax

SQL

SELECT DISTINCT column_name FROM table_name;

SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name;

SELECT column_name FROM table_name ORDER BY column_name [ASC|DESC];

NULL values are treated as equal in DISTINCT and GROUP BY, so they appear as one group or one distinct value.

In ORDER BY, NULLs usually appear first in ascending order and last in descending order, but this can vary by database.

Examples

This finds unique cities, treating all NULL cities as one group.

SQL

SELECT DISTINCT city FROM customers;

This counts how many customers are in each city, including those with NULL city.

SQL

SELECT city, COUNT(*) FROM customers GROUP BY city;

This sorts customers by city name, with NULL cities usually appearing first.

SQL

SELECT city FROM customers ORDER BY city ASC;

Sample Program

This example shows how NULL values appear in DISTINCT, GROUP BY, and ORDER BY results.

SQL

CREATE TABLE customers (id INT, city VARCHAR(20));
INSERT INTO customers VALUES (1, 'New York'), (2, NULL), (3, 'Los Angeles'), (4, NULL), (5, 'New York');

-- Find distinct cities
SELECT DISTINCT city FROM customers;

-- Count customers per city
SELECT city, COUNT(*) FROM customers GROUP BY city;

-- Order customers by city
SELECT city FROM customers ORDER BY city ASC;

Important Notes

NULL is not the same as an empty string or zero; it means unknown or missing.

Some databases allow controlling NULL order in ORDER BY with NULLS FIRST or NULLS LAST.

Summary

NULL values are grouped together as one in DISTINCT and GROUP BY.

ORDER BY usually places NULLs before other values in ascending order.

Knowing how NULL behaves helps avoid surprises in data results.

Practice

1. What happens to NULL values when using DISTINCT in SQL?

easy

A. NULL values are ignored and not shown in the result.

B. All NULL values are treated as one group and appear once.

C. Each NULL is treated as a unique value and appears multiple times.

D. DISTINCT causes an error if NULL values exist.

Solution

1. Step 1: Understand DISTINCT behavior with NULL

   In SQL, DISTINCT treats all NULL values as equal, grouping them together.

2. Step 2: Result of DISTINCT on NULL values

   This means only one NULL appears in the output, no matter how many NULLs exist.

3. Final Answer:

   All NULL values are treated as one group and appear once. -> Option B

4. Quick Check:

   NULLs grouped once = A [OK]

Hint: NULLs count as one in DISTINCT and GROUP BY [OK]

Common Mistakes:

• Thinking NULLs are ignored in DISTINCT
• Assuming each NULL is unique
• Believing DISTINCT errors on NULL

2. Which of the following is the correct syntax to select distinct values including NULLs from a table named employees on column department?

easy

A. SELECT department DISTINCT FROM employees;

B. SELECT UNIQUE department FROM employees;

C. SELECT DISTINCT(department) FROM employees;

D. SELECT DISTINCT department FROM employees;

Solution

1. Step 1: Identify correct DISTINCT syntax

   The correct SQL syntax to get unique values is SELECT DISTINCT column FROM table;. Parentheses are not used with DISTINCT.

2. Step 2: Check each option

   SELECT DISTINCT department FROM employees; uses correct syntax. SELECT UNIQUE department FROM employees; uses UNIQUE which is not standard SQL. SELECT DISTINCT(department) FROM employees; incorrectly uses parentheses. SELECT department DISTINCT FROM employees; has wrong keyword order.

3. Final Answer:

   SELECT DISTINCT department FROM employees; -> Option D

4. Quick Check:

   Standard DISTINCT syntax = D [OK]

Hint: Use DISTINCT without parentheses for unique values [OK]

Common Mistakes:

• Using UNIQUE instead of DISTINCT
• Adding parentheses after DISTINCT
• Misplacing DISTINCT keyword

3. Given the table sales with column region containing values: ('East', 'West', NULL, 'East', NULL), what will be the result of this query?

SELECT region FROM sales GROUP BY region ORDER BY region ASC;

medium

A. [NULL, 'East', 'West']

B. ['East', 'West', NULL]

C. ['East', NULL, 'West']

D. [NULL, 'West', 'East']

Solution

1. Step 1: GROUP BY groups NULLs together

   The GROUP BY groups all NULL values as one group. So NULL appears once.

2. Step 2: ORDER BY ascending places NULL first

   In ascending order, NULL values come before any text values. Then 'East' and 'West' follow alphabetically.

3. Final Answer:

   [NULL, 'East', 'West'] -> Option A

4. Quick Check:

   NULL first in ORDER BY ASC = A [OK]

Hint: ORDER BY ASC puts NULL before text values [OK]

Common Mistakes:

• Assuming NULL sorts last
• Treating NULL as a string value
• Expecting multiple NULL rows in GROUP BY

4. What is wrong with this query if the goal is to list unique category values including NULLs?

SELECT category FROM products GROUP BY category ORDER BY category;

medium

A. Query is correct and will work as intended.

B. ORDER BY cannot be used with GROUP BY.

C. Missing closing semicolon causes syntax error.

D. GROUP BY ignores NULL values, so NULLs won't appear.

Solution

1. Step 1: Check syntax and logic

   The query groups by category and orders by category. This is valid SQL syntax.

2. Step 2: Understand NULL handling

   GROUP BY includes NULL as a group, so NULL values appear once. ORDER BY sorts NULLs first by default.

3. Final Answer:

   Query is correct and will work as intended. -> Option A

4. Quick Check:

   GROUP BY with ORDER BY and NULLs works = C [OK]

Hint: GROUP BY includes NULLs; ORDER BY works with GROUP BY [OK]

Common Mistakes:

• Thinking NULLs are excluded by GROUP BY
• Believing ORDER BY can't follow GROUP BY
• Assuming missing semicolon breaks query

5. You have a table orders with a column status that can be 'Pending', 'Shipped', or NULL. You want to count how many orders exist for each status, including NULLs as a separate group. Which query correctly achieves this?

hard

A. SELECT status, COUNT(*) FROM orders GROUP BY status HAVING status IS NOT NULL ORDER BY status;

B. SELECT status, COUNT(*) FROM orders WHERE status IS NOT NULL GROUP BY status ORDER BY status;

C. SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY status ASC;

D. SELECT DISTINCT status, COUNT(*) FROM orders ORDER BY status;

Solution

1. Step 1: Understand grouping with NULL

   GROUP BY status groups NULL values as one group, so counting includes NULLs separately.

2. Step 2: Analyze each option

   SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY status ASC; groups all statuses including NULL and orders them ascending. SELECT status, COUNT(*) FROM orders GROUP BY status HAVING status IS NOT NULL ORDER BY status; excludes NULL with HAVING. SELECT status, COUNT(*) FROM orders WHERE status IS NOT NULL GROUP BY status ORDER BY status; filters NULL out in WHERE. SELECT DISTINCT status, COUNT(*) FROM orders ORDER BY status; uses DISTINCT with COUNT(*) incorrectly.

3. Final Answer:

   SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY status ASC; -> Option C

4. Quick Check:

   GROUP BY includes NULL groups = B [OK]

Hint: Use GROUP BY without WHERE to include NULL counts [OK]

Common Mistakes:

• Filtering NULLs out before grouping
• Using HAVING to exclude NULLs unintentionally
• Misusing DISTINCT with aggregate functions