Percent of total with window functions in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the percent of total sales for each product.

SQL

SELECT product_id, sales_amount, sales_amount [1] SUM(sales_amount) OVER () AS percent_of_total FROM sales;

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

A+

B*

C/

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of division.

Forgetting to use the window function SUM() OVER ().

We divide each sales_amount by the total sales (SUM over all rows) to get the percent of total.

2fill in blank

medium

Complete the code to calculate the percent of total sales rounded to two decimal places.

SQL

SELECT product_id, ROUND(sales_amount [1] SUM(sales_amount) OVER () * 100, 2) AS percent_of_total FROM sales;

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

A/

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or subtraction instead of division.

Multiplying before dividing.

We divide sales_amount by total sales, multiply by 100 to get a percentage, then round to 2 decimals.

3fill in blank

hard

Fix the error in the code to correctly calculate the percent of total sales per category.

SQL

SELECT category, sales_amount, sales_amount [1] SUM(sales_amount) OVER (PARTITION BY category) AS percent_of_category_total FROM sales;

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or multiplication instead of division.

Not using PARTITION BY in the window function.

To get the percent of total per category, divide sales_amount by the sum of sales_amount partitioned by category.

4fill in blank

hard

Fill both blanks to calculate the percent of total sales and alias the result as percent_total.

SQL

SELECT product_id, sales_amount, (sales_amount [1] SUM(sales_amount) OVER ()) * 100 AS [2] FROM sales;

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

A/

B*

Cpercent_total

Dpercent

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of division.

Using unclear or missing alias names.

Divide sales_amount by total sales, multiply by 100, and name the column percent_total.

5fill in blank

hard

Fill all three blanks to calculate the percent of total sales per region, rounded to 1 decimal place, and alias it as pct_region.

SQL

SELECT region, sales_amount, ROUND((sales_amount [1] SUM(sales_amount) OVER (PARTITION BY region)) * [2], [3]) AS pct_region FROM sales;

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

A/

B100

C1

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting PARTITION BY region.

Forgetting to multiply by 100.

Rounding to wrong decimal places.

Divide sales_amount by total sales per region, multiply by 100 to get percent, then round to 1 decimal place.

Practice

1. What does the SQL window function SUM(column) OVER () compute in a query?

easy

A. The maximum value of the column

B. The total sum of the column across all rows without grouping

C. The average of the column values

D. The sum of the column for each group defined by GROUP BY

Solution

1. Step 1: Understand window function syntax

   The syntax SUM(column) OVER () means sum over all rows without partitioning or grouping.

2. Step 2: Compare with GROUP BY sum

   Unlike GROUP BY, this sums the entire column across all rows, returning the same total for each row.

3. Final Answer:

   The total sum of the column across all rows without grouping -> Option B

4. Quick Check:

   SUM() OVER () = total sum [OK]

Hint: SUM() OVER () gives total sum for all rows, no grouping [OK]

Common Mistakes:

• Confusing window sum with GROUP BY sum
• Thinking it calculates average
• Assuming it returns max value

2. Which of the following SQL queries correctly calculates the percent of total sales per product using window functions?

easy

A. SELECT product, sales, (sales / SUM(sales) OVER ()) * 100 AS percent_total FROM sales_table;

B. SELECT product, sales, (SUM(sales) OVER () / sales) * 100 AS percent_total FROM sales_table;

C. SELECT product, sales, (sales / SUM(sales)) * 100 AS percent_total FROM sales_table GROUP BY product;

D. SELECT product, sales, (sales / COUNT(*) OVER ()) * 100 AS percent_total FROM sales_table;

Solution

1. Step 1: Identify correct window function usage

   SELECT product, sales, (sales / SUM(sales) OVER ()) * 100 AS percent_total FROM sales_table; uses SUM(sales) OVER () to get total sales across all rows, then divides each sales value by this total.

2. Step 2: Check other options for errors

   SELECT product, sales, (SUM(sales) OVER () / sales) * 100 AS percent_total FROM sales_table; divides total by sales (wrong direction). SELECT product, sales, (sales / SUM(sales)) * 100 AS percent_total FROM sales_table GROUP BY product; uses SUM(sales) without OVER(), which requires GROUP BY and won't work as intended. SELECT product, sales, (sales / COUNT(*) OVER ()) * 100 AS percent_total FROM sales_table; divides sales by count of rows, not sum.

3. Final Answer:

   SELECT product, sales, (sales / SUM(sales) OVER ()) * 100 AS percent_total FROM sales_table; -> Option A

4. Quick Check:

   Percent = sales / total sales * 100 [OK]

Hint: Divide sales by SUM() OVER () and multiply by 100 [OK]

Common Mistakes:

• Reversing numerator and denominator
• Using SUM() without OVER() in window context
• Using COUNT() instead of SUM() for total

3. Given the table sales_data with columns product and amount, what is the output of this query?

SELECT product, amount, ROUND((amount / SUM(amount) OVER ()) * 100, 1) AS percent_total FROM sales_data ORDER BY product;

Assume the data:
product | amount
-------|-------
A | 50
B | 30
C | 20

medium

A. [{"product": "A", "amount": 50, "percent_total": 100.0}, {"product": "B", "amount": 30, "percent_total": 100.0}, {"product": "C", "amount": 20, "percent_total": 100.0}]

B. [{"product": "A", "amount": 50, "percent_total": 33.3}, {"product": "B", "amount": 30, "percent_total": 20.0}, {"product": "C", "amount": 20, "percent_total": 13.3}]

C. [{"product": "A", "amount": 50, "percent_total": 25.0}, {"product": "B", "amount": 30, "percent_total": 15.0}, {"product": "C", "amount": 20, "percent_total": 10.0}]

D. [{"product": "A", "amount": 50, "percent_total": 50.0}, {"product": "B", "amount": 30, "percent_total": 30.0}, {"product": "C", "amount": 20, "percent_total": 20.0}]

Solution

1. Step 1: Calculate total amount

   Total = 50 + 30 + 20 = 100.

2. Step 2: Calculate percent for each product

   Percent = (amount / total) * 100 rounded to 1 decimal:
   A: (50/100)*100 = 50.0
   B: (30/100)*100 = 30.0
   C: (20/100)*100 = 20.0

3. Final Answer:

   [{"product": "A", "amount": 50, "percent_total": 50.0}, {"product": "B", "amount": 30, "percent_total": 30.0}, {"product": "C", "amount": 20, "percent_total": 20.0}] -> Option D

4. Quick Check:

   Percent totals sum to 100% [OK]

Hint: Sum amounts, divide each by total, multiply by 100 [OK]

Common Mistakes:

• Not rounding results
• Using COUNT() instead of SUM()
• Calculating percent incorrectly (e.g., reversed division)

4. You wrote this query to calculate percent of total sales:

SELECT product, sales, (sales / SUM(sales)) * 100 AS percent_total FROM sales_table;

But it gives an error. What is the problem?

medium

A. SUM(sales) requires a GROUP BY clause or window function syntax

B. sales column does not exist

C. Multiplying by 100 is invalid syntax

D. Missing WHERE clause

Solution

1. Step 1: Analyze use of SUM() without OVER()

   SUM(sales) without GROUP BY or window function context causes error because SQL doesn't know how to aggregate without grouping.

2. Step 2: Correct usage

   Use either GROUP BY product with SUM(sales) or use window function SUM(sales) OVER () to get total sum without grouping.

3. Final Answer:

   SUM(sales) requires a GROUP BY clause or window function syntax -> Option A

4. Quick Check:

   SUM() needs GROUP BY or OVER() [OK]

Hint: Use SUM() OVER () or GROUP BY with SUM() [OK]

Common Mistakes:

• Ignoring need for GROUP BY with aggregate functions
• Assuming multiplication by 100 causes error
• Thinking WHERE clause is mandatory here

5. You have a table orders with columns category and amount. Write a query to show each category, total amount per category, and percent of total amount across all categories. Which query is correct?

hard

A. SELECT category, SUM(amount) AS category_total, (SUM(amount) / COUNT(*) OVER ()) * 100 AS percent_total FROM orders GROUP BY category;

B. SELECT category, amount, (amount / SUM(amount) OVER ()) * 100 AS percent_total FROM orders ORDER BY category;

C. SELECT category, SUM(amount) AS category_total, (SUM(amount) / SUM(SUM(amount)) OVER ()) * 100 AS percent_total FROM orders GROUP BY category ORDER BY category;

D. SELECT category, SUM(amount) AS category_total, (SUM(amount) / SUM(amount)) * 100 AS percent_total FROM orders GROUP BY category;

Solution

1. Step 1: Calculate total per category

   Use SUM(amount) GROUP BY category to get total per category.

2. Step 2: Calculate percent of total across all categories

   Use window function SUM(SUM(amount)) OVER () to get grand total across all categories, then divide category total by this and multiply by 100.

3. Step 3: Verify query correctness

   SELECT category, SUM(amount) AS category_total, (SUM(amount) / SUM(SUM(amount)) OVER ()) * 100 AS percent_total FROM orders GROUP BY category ORDER BY category; correctly combines GROUP BY with window function for total sum. SELECT category, amount, (amount / SUM(amount) OVER ()) * 100 AS percent_total FROM orders ORDER BY category; uses amount per row, not category total. SELECT category, SUM(amount) AS category_total, (SUM(amount) / COUNT(*) OVER ()) * 100 AS percent_total FROM orders GROUP BY category; incorrectly uses COUNT() for total. SELECT category, SUM(amount) AS category_total, (SUM(amount) / SUM(amount)) * 100 AS percent_total FROM orders GROUP BY category; divides sum by itself, always 100%.

4. Final Answer:

   SELECT category, SUM(amount) AS category_total, (SUM(amount) / SUM(SUM(amount)) OVER ()) * 100 AS percent_total FROM orders GROUP BY category ORDER BY category; -> Option C

5. Quick Check:

   Percent = category sum / total sum * 100 [OK]

Hint: Use GROUP BY with SUM() and window SUM(SUM(amount)) OVER () for total [OK]

Common Mistakes:

• Using row amount instead of category sum
• Using COUNT() instead of SUM() for total
• Dividing sum by itself always gives 100%