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.