NULL in DISTINCT, GROUP BY, and ORDER BY in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query to select unique values including NULLs from the column 'category'.

SQL

SELECT DISTINCT [1] FROM products;

Drag options to blanks, or click blank then click option'

Acategory

Bcategory IS NOT NULL

CCOUNT(category)

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' with DISTINCT returns unique rows, not unique values of a single column.

Using COUNT(category) returns a count, not distinct values.

Using 'category IS NOT NULL' is a condition, not a column name.

The DISTINCT keyword selects unique values including NULLs from the specified column. Using 'category' returns unique categories including NULL.

2fill in blank

medium

Complete the SQL query to group rows by the 'status' column, including NULL values as a group.

SQL

SELECT status, COUNT(*) FROM orders GROUP BY [1];

Drag options to blanks, or click blank then click option'

Astatus IS NOT NULL

BCOUNT(status)

Cstatus

DDISTINCT status

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(status) in GROUP BY causes syntax error.

Using 'status IS NOT NULL' is a condition, not a grouping column.

Using DISTINCT status is invalid in GROUP BY clause.

GROUP BY groups rows by the column values including NULLs. Using 'status' groups all rows by their status values, including NULL as its own group.

3fill in blank

hard

Fix the error in the ORDER BY clause to sort by 'price' including NULL values last.

SQL

SELECT * FROM products ORDER BY price [1];

Drag options to blanks, or click blank then click option'

ADESC

BASC NULLS LAST

CDESC NULLS FIRST

DASC

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC sorts descending, not ascending.

Using NULLS FIRST places NULLs before numbers.

Omitting NULLS LAST places NULLs first by default in some databases.

To sort prices ascending with NULL values at the end, use 'ASC NULLS LAST'. This ensures NULLs appear after all numbers.

4fill in blank

hard

Fill both blanks to select unique 'department' values and order them with NULLs first.

SQL

SELECT DISTINCT [1] FROM employees ORDER BY [2] NULLS FIRST;

Drag options to blanks, or click blank then click option'

Adepartment

Bsalary

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by a different column than selected causes confusing results.

Using salary or employee_id does not match the selected column.

Omitting NULLS FIRST places NULLs last by default.

We select DISTINCT 'department' and order by 'department' with NULLs first to see NULL departments at the top.

5fill in blank

hard

Fill all three blanks to group by 'region', count orders, and order groups with NULL regions last.

SQL

SELECT [1], COUNT(*) FROM sales GROUP BY [2] ORDER BY [3] NULLS LAST;

Drag options to blanks, or click blank then click option'

Aregion

Dorder_date

Attempts:

3 left

💡 Hint

Common Mistakes

Using different columns for select, group by, and order by causes errors or confusing results.

Ordering by order_date does not match grouping by region.

Omitting NULLS LAST places NULLs first by default in some systems.

We select and group by 'region' to count orders per region, then order by 'region' placing 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