GROUP BY with NULL values behavior in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of GROUP BY with NULL values

Consider the table Employees with columns Department and Salary. Some Department values are NULL.
What is the output of this query?

SELECT Department, COUNT(*) AS Count FROM Employees GROUP BY Department ORDER BY Department;

SQL

CREATE TABLE Employees (Department VARCHAR(20), Salary INT);
INSERT INTO Employees VALUES
(NULL, 5000), ('HR', 4000), ('IT', 6000), (NULL, 4500), ('HR', 4200);

A [{"Department": "HR", "Count": 2}, {"Department": "IT", "Count": 1}, {"Department": null, "Count": 0}]

B [{"Department": "HR", "Count": 2}, {"Department": "IT", "Count": 1}]

C [{"Department": null, "Count": 2}, {"Department": "HR", "Count": 2}, {"Department": "IT", "Count": 1}]

D [{"Department": "HR", "Count": 2}, {"Department": "IT", "Count": 1}, {"Department": "NULL", "Count": 2}]

💡 Hint

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

In SQL, NULL values in GROUP BY are treated as a single group. So all rows with Department NULL are counted together.

🧠 Conceptual

intermediate

How does GROUP BY treat NULL values?

In SQL, when you use GROUP BY on a column that contains NULL values, how are those NULL values treated?

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

B Each NULL value is treated as a distinct group.

C NULL values cause the query to fail with an error.

D All NULL values are grouped together as a single group.

💡 Hint

Think about how NULL represents unknown but equal unknowns in grouping.

SQL treats all NULLs in a GROUP BY column as one group, so they are counted together.

📝 Syntax

advanced

Identify the error in GROUP BY with NULL handling

Which of the following SQL queries will cause a syntax error when grouping by a column that contains NULL values?

SQL

Table: Sales (Region VARCHAR, Amount INT) with some NULL Region values

A SELECT Region, SUM(Amount) FROM Sales GROUP BY Region;

B SELECT Region, SUM(Amount) FROM Sales GROUP BY Region WHERE Region IS NOT NULL;

C SELECT Region, SUM(Amount) FROM Sales GROUP BY Region HAVING Region IS NOT NULL;

D SELECT Region, SUM(Amount) FROM Sales GROUP BY Region ORDER BY Region;

💡 Hint

Remember the order of clauses in SQL SELECT statements.

The WHERE clause cannot come after GROUP BY; it must come before. Using WHERE after GROUP BY causes syntax error.

❓ optimization

advanced

Optimizing GROUP BY with NULL values filtering

You want to count rows per category but exclude NULL categories efficiently. Which query is best for performance?

A SELECT Category, COUNT(*) FROM Products WHERE Category IS NOT NULL GROUP BY Category;

B SELECT Category, COUNT(*) FROM Products GROUP BY Category HAVING Category IS NOT NULL;

C SELECT Category, COUNT(*) FROM Products GROUP BY Category;

D SELECT Category, COUNT(*) FROM Products WHERE Category <> NULL GROUP BY Category;

💡 Hint

Filtering rows before grouping reduces data processed.

Filtering NULLs in WHERE clause reduces rows before grouping, improving performance over HAVING.

🔧 Debug

expert

Why does this GROUP BY query return unexpected NULL group?

Given the table Orders with columns CustomerID and Status, the query below returns a group with NULL CustomerID unexpectedly.

SELECT CustomerID, COUNT(*) FROM Orders GROUP BY CustomerID;

What is the most likely reason for this NULL group appearing?

A There are rows where CustomerID is NULL, and GROUP BY treats all NULLs as one group.

B The query is missing a WHERE clause to exclude NULL CustomerIDs, causing a syntax error.

C CustomerID column has a default value of NULL which is not allowed in GROUP BY.

D The database automatically converts NULLs to zero in GROUP BY, causing confusion.

💡 Hint

Check if any rows have NULL in the grouped column.

GROUP BY groups all NULL values together, so if CustomerID is NULL in some rows, they form a NULL group.

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