GROUP BY with NULL values behavior in SQL

Introduction

We use GROUP BY to group rows that have the same values in specified columns. Understanding how NULL values behave helps us group data correctly when some values are missing.

You want to count or summarize data by categories that might have missing values.

You need to group sales data by product category, but some products have no category assigned (NULL).

You want to see how many users belong to each department, including those without a department (NULL).

You want to group survey answers where some responses are missing (NULL).

Syntax

SQL

SELECT column1, aggregate_function(column2)
FROM table_name
GROUP BY column1;

NULL values in the GROUP BY column are treated as a single group.

All rows with NULL in the grouped column are grouped together.

Examples

This groups employees by their department. Employees with NULL department are grouped together.

SQL

SELECT department, COUNT(*)
FROM employees
GROUP BY department;

This sums sales by product category. Products with NULL category are grouped as one group.

SQL

SELECT category, SUM(sales)
FROM products
GROUP BY category;

Sample Program

This example creates an employees table with some NULL departments. It groups employees by department, counting how many are in each group. NULL departments are grouped together.

SQL

CREATE TABLE employees (
  id INT,
  name VARCHAR(50),
  department VARCHAR(50)
);

INSERT INTO employees (id, name, department) VALUES
(1, 'Alice', 'HR'),
(2, 'Bob', NULL),
(3, 'Charlie', 'IT'),
(4, 'David', NULL),
(5, 'Eve', 'HR');

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department
ORDER BY department;

Important Notes

NULL values are grouped together as one group in GROUP BY.

Ordering by the grouped column may show NULL as empty or at the start/end depending on the database.

Summary

GROUP BY treats all NULL values in a column as one group.

This helps include rows with missing values in grouped results.

Practice

1. What happens to NULL values when you use GROUP BY on a column containing them?

easy

A. All NULL values are grouped together as one group.

B. NULL values are ignored and not included in any group.

C. Each NULL value forms its own separate group.

D. GROUP BY causes an error if NULL values exist.

Solution

1. Step 1: Understand how GROUP BY handles NULLs

   In SQL, GROUP BY treats all NULL values in a column as equal, grouping them into one group.

2. Step 2: Confirm behavior with example

   If a column has multiple rows with NULL, they appear as a single group in the result.

3. Final Answer:

   All NULL values are grouped together as one group. -> Option A

4. Quick Check:

   GROUP BY NULL = one group [OK]

Hint: Remember: NULLs group together, not separately [OK]

Common Mistakes:

• Thinking NULLs are ignored in GROUP BY
• Assuming each NULL is a separate group
• Believing GROUP BY errors on NULL values

2. Which of the following SQL queries correctly groups rows by a column that may contain NULL values?

easy

A. SELECT category, COUNT(*) FROM products GROUP BY category;

B. SELECT category, COUNT(*) FROM products GROUP BY category WHERE category IS NOT NULL;

C. SELECT category, COUNT(*) FROM products WHERE category IS NOT NULL GROUP BY category;

D. SELECT category, COUNT(*) FROM products GROUP BY category HAVING category IS NOT NULL;

Solution

1. Step 1: Check GROUP BY syntax with NULLs

   SELECT category, COUNT(*) FROM products GROUP BY category; uses correct syntax: grouping by category including NULLs. GROUP BY works with NULL values without extra filters.

2. Step 2: Analyze other options

   Options A and D misuse WHERE and HAVING clauses with GROUP BY. SELECT category, COUNT(*) FROM products WHERE category IS NOT NULL GROUP BY category; filters out NULLs before grouping, which is valid but excludes NULL groups.

3. Final Answer:

   SELECT category, COUNT(*) FROM products GROUP BY category; -> Option A

4. Quick Check:

   GROUP BY with NULLs needs no special filter [OK]

Hint: GROUP BY works directly with NULLs, no WHERE needed [OK]

Common Mistakes:

• Using WHERE after GROUP BY (syntax error)
• Filtering NULLs before grouping unintentionally
• Misusing HAVING clause for filtering NULLs

3. Given the table sales with data:

product | region
-------|--------
A      | East
B      | NULL
A      | NULL
B      | East
NULL   | West
NULL   | NULL

What is the result of:

SELECT region, COUNT(*) FROM sales GROUP BY region ORDER BY region;

medium

A. [
{"region": "East", "count": 2},
{"region": "NULL", "count": 3},
{"region": "West", "count": 1}
]

B. [
{"region": "East", "count": 2},
{"region": "NULL", "count": 2},
{"region": "West", "count": 1}
]

C. [
{"region": "East", "count": 2},
{"region": null, "count": 2},
{"region": "West", "count": 1}
]

D. [
{"region": null, "count": 3},
{"region": "East", "count": 2},
{"region": "West", "count": 1}
]

Solution

1. Step 1: Group rows by region including NULLs

   Rows with region 'East' = 2, 'West' = 1, and NULL values (3 rows) are grouped together as one NULL group.

2. Step 2: Understand NULL display and count

   SQL returns NULL as null (not string 'NULL'). Count for NULL group is 3 because three rows have region NULL. ORDER BY region ASC places null first.

3. Final Answer:

   [{"region": null, "count": 3}, {"region": "East", "count": 2}, {"region": "West", "count": 1}] -> Option D

4. Quick Check:

   GROUP BY NULL groups count 3, NULL shown as null [OK]

Hint: NULLs group together and show as null, not 'NULL' string [OK]

Common Mistakes:

• Counting NULL rows separately
• Displaying NULL as string 'NULL'
• Miscounting NULL group size

4. Consider this query:

SELECT department, COUNT(*) FROM employees GROUP BY department;

It returns an error. Which fix will correctly handle NULL values in department to avoid errors?

medium

A. Add WHERE department IS NOT NULL before GROUP BY.

B. No fix needed; GROUP BY never errors on NULL.

C. Use HAVING department IS NOT NULL after GROUP BY.

D. Replace NULL with a string using COALESCE(department, 'Unknown') in SELECT and GROUP BY.

Solution

1. Step 1: Understand why no error occurs

   In standard SQL, GROUP BY handles NULL values correctly by grouping all NULLs together into one group. No error is thrown.

2. Step 2: Confirm no fix needed

   The query runs successfully and includes a NULL group in the results.

3. Final Answer:

   No fix needed; GROUP BY never errors on NULL. -> Option B

4. Quick Check:

   GROUP BY NULL = no error [OK]

Hint: GROUP BY handles NULLs without error [OK]

Common Mistakes:

• Thinking GROUP BY errors on NULLs
• Unnecessarily filtering out NULLs with WHERE
• Misusing HAVING for pre-group filtering

5. You have a table orders with columns customer_id and status, where status can be NULL. You want to count orders by status, treating all NULL statuses as 'Pending'. Which query correctly achieves this?

hard

A. SELECT status, COUNT(*) FROM orders GROUP BY status WHERE status IS NULL;

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

C. SELECT COALESCE(status, 'Pending') AS order_status, COUNT(*) FROM orders GROUP BY order_status;

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

Solution

1. Step 1: Replace NULL with 'Pending' using COALESCE

   COALESCE(status, 'Pending') converts NULL statuses to 'Pending' for counting.

2. Step 2: Group by the alias used in SELECT

   Grouping by order_status ensures all NULLs are counted under 'Pending'.

3. Final Answer:

   SELECT COALESCE(status, 'Pending') AS order_status, COUNT(*) FROM orders GROUP BY order_status; -> Option C

4. Quick Check:

   COALESCE + GROUP BY alias counts NULL as 'Pending' [OK]

Hint: Use COALESCE and group by alias to count NULLs as desired [OK]

Common Mistakes:

• Filtering out NULLs instead of replacing them
• Using WHERE after GROUP BY (syntax error)
• Grouping by original column without COALESCE