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;

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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];

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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];

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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;

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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.