NULL in DISTINCT, GROUP BY, and ORDER BY in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of DISTINCT with NULL values

Consider a table Employees with a column Department that contains some NULL values. What will be the result of the following query?

SELECT DISTINCT Department FROM Employees ORDER BY Department;

SQL

CREATE TABLE Employees (ID INT, Department VARCHAR(20));
INSERT INTO Employees VALUES (1, 'Sales'), (2, NULL), (3, 'HR'), (4, NULL), (5, 'Sales');

SELECT DISTINCT Department FROM Employees ORDER BY Department;

A NULL appears once at the top, followed by 'HR' and 'Sales' in alphabetical order

B NULL appears multiple times, one for each NULL row, followed by 'HR' and 'Sales'

C 'HR' and 'Sales' appear, but NULL values are excluded

D Query results in an error because NULL cannot be ordered

💡 Hint

DISTINCT removes duplicates including NULLs, and ORDER BY sorts NULLs first by default in many SQL dialects.

The DISTINCT keyword treats all NULLs as one group, so only one NULL appears. ORDER BY sorts NULLs before non-NULL values by default, so NULL appears first, then 'HR' and 'Sales' alphabetically.

❓ query_result

intermediate

GROUP BY behavior with NULL values

Given a table Orders with a column Region that contains NULL values, what will be the output of this query?

SELECT Region, COUNT(*) FROM Orders GROUP BY Region ORDER BY Region;

SQL

CREATE TABLE Orders (OrderID INT, Region VARCHAR(20));
INSERT INTO Orders VALUES (1, 'East'), (2, NULL), (3, 'West'), (4, NULL), (5, 'East');

SELECT Region, COUNT(*) FROM Orders GROUP BY Region ORDER BY Region;

A Two rows with NULL and 'East' only, 'West' is excluded

B Three rows: NULL, 'East', and 'West', with NULL last in order

C Three rows: NULL, 'East', and 'West', with NULL first in order

D Query fails because NULL cannot be grouped

💡 Hint

GROUP BY treats NULL as a group, and ORDER BY sorts NULLs first by default.

GROUP BY groups NULL values together as one group. ORDER BY sorts NULLs before non-NULL values, so the output has three rows with NULL first, then 'East', then 'West'.

📝 Syntax

advanced

Ordering NULLs explicitly in ORDER BY

Which of the following queries will correctly order the Category column with NULLs appearing last?

SELECT Category FROM Products ORDER BY Category NULLS LAST;

A SELECT Category FROM Products ORDER BY Category NULLS LAST;

B SELECT Category FROM Products ORDER BY Category NULL LAST;

C SELECT Category FROM Products ORDER BY Category IS NULL, Category;

D SELECT Category FROM Products ORDER BY Category DESC NULLS LAST;

💡 Hint

The standard syntax to put NULLs last is 'NULLS LAST' after the column name.

Option A uses the correct standard SQL syntax to order NULLs last. Option A is valid but orders descending first, which changes the order. Option A is a workaround but not standard syntax. Option A is invalid syntax.

❓ optimization

advanced

Optimizing GROUP BY with NULL values

You want to optimize a query that groups by a column Status which contains many NULL values. Which approach can improve performance while keeping NULLs grouped correctly?

A Use DISTINCT instead of GROUP BY to avoid NULL grouping overhead

B Replace NULLs with a placeholder value using COALESCE(Status, 'Unknown') in GROUP BY

C Use GROUP BY Status without changes; NULLs are grouped automatically

D Exclude NULL rows using WHERE Status IS NOT NULL before GROUP BY

💡 Hint

Replacing NULLs with a fixed value can help indexes and grouping.

Using COALESCE replaces NULLs with a known value, which can improve grouping performance and indexing. Excluding NULLs changes results. GROUP BY alone works but may be slower. DISTINCT does not replace GROUP BY functionality.

🧠 Conceptual

expert

Understanding NULL behavior in DISTINCT, GROUP BY, and ORDER BY

Which statement correctly describes how NULL values behave in DISTINCT, GROUP BY, and ORDER BY clauses in SQL?

A NULLs cause errors in DISTINCT and GROUP BY, but ORDER BY ignores NULLs

B NULLs are treated as distinct in DISTINCT, grouped together in GROUP BY, and sorted first in ORDER BY by default in many SQL dialects

C NULLs are treated as equal in DISTINCT and GROUP BY, but ORDER BY always places NULLs at the end regardless of dialect

D NULLs are treated as equal in DISTINCT and GROUP BY, and ORDER BY placement of NULLs depends on the SQL dialect and can be controlled explicitly

💡 Hint

Think about how NULLs are grouped and sorted in different SQL clauses and dialects.

NULLs are treated as equal for DISTINCT and GROUP BY, so they appear as one group. ORDER BY placement of NULLs varies by SQL dialect; some put NULLs first, others last. This can be controlled explicitly with NULLS FIRST or NULLS LAST.

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