GROUP BY with NULL values behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to group rows by the column that may contain NULL values.

SQL

SELECT department, COUNT(*) FROM employees GROUP BY [1];

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

Ahire_date

Bsalary

Cemployee_id

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a column not in the SELECT list for grouping.

Using a column that does not relate to grouping by department.

The GROUP BY clause groups rows by the specified column. Here, grouping by department groups employees by their department, including those with NULL values.

2fill in blank

medium

Complete the code to count how many rows have NULL in the department column.

SQL

SELECT COUNT(*) FROM employees WHERE department [1] NULL;

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

A=

BIS

C!=

DLIKE

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' to compare with NULL.

Using 'LIKE' which is for pattern matching.

To check for NULL values in SQL, use IS NULL. Using = NULL does not work because NULL means unknown.

3fill in blank

hard

Fix the error in the GROUP BY clause to correctly group by department including NULLs.

SQL

SELECT department, COUNT(*) FROM employees GROUP BY [1];

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

ACOUNT(department)

Bdepartment IS NULL

Cdepartment

Ddepartment = NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using conditions like 'department IS NULL' in GROUP BY.

Using aggregate functions in GROUP BY.

The GROUP BY clause must use the column name directly. Expressions like department IS NULL or comparisons with NULL are invalid here.

4fill in blank

hard

Fill both blanks to group by department and replace NULL with 'Unknown' in the output.

SQL

SELECT COALESCE([1], 'Unknown') AS dept, COUNT(*) FROM employees GROUP BY [2];

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

Adepartment

Bsalary

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using different columns in COALESCE and GROUP BY.

Trying to group by the COALESCE expression instead of the column.

COALESCE(department, 'Unknown') replaces NULL with 'Unknown' in the output. The GROUP BY must use the original column department to group rows correctly.

5fill in blank

hard

Fill all three blanks to count employees grouped by department, showing 'No Dept' for NULLs and ordering by count descending.

SQL

SELECT COALESCE([1], [2]) AS dept, COUNT(*) AS total FROM employees GROUP BY [3] ORDER BY total DESC;

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

Adepartment

B'No Dept'

Dsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by the COALESCE expression instead of the column.

Not replacing NULLs in the SELECT output.

Ordering by the wrong column.

Use COALESCE(department, 'No Dept') to replace NULLs with 'No Dept'. Group by the original department column. Ordering by total DESC shows groups with most employees first.

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