Running totals with SUM OVER in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SUM() OVER() function do in SQL?

It calculates a running total or cumulative sum of values over a specified order without collapsing rows.

beginner

How do you specify the order for a running total using SUM() OVER()?

You use the ORDER BY clause inside the OVER() clause to define the order of rows for the running total.

intermediate

What is the difference between SUM() OVER() and a regular SUM() with GROUP BY?

SUM() OVER() returns a running total for each row, keeping all rows visible. Regular SUM() with GROUP BY aggregates rows into groups, returning one row per group.

beginner

Write a simple SQL query to calculate a running total of sales ordered by date.

SELECT date, sales, SUM(sales) OVER (ORDER BY date) AS running_total FROM sales_table;

intermediate

Can SUM() OVER() be used without an ORDER BY clause? What happens?

Yes, but without ORDER BY, it sums over the entire partition and returns the same total for every row, not a running total.

What does the ORDER BY clause inside SUM() OVER() control?

A The filtering of rows before summing

B The order of the final query result only

C The order in which rows are summed for the running total

D The grouping of rows for aggregation

The ORDER BY inside OVER() defines the sequence in which rows are processed to calculate the running total.

Which SQL clause is necessary to calculate a running total with SUM() OVER()?

A HAVING

B GROUP BY

C WHERE

D ORDER BY inside OVER()

The running total requires ORDER BY inside the OVER() clause to define the order of accumulation.

What will SUM(sales) OVER () without ORDER BY return?

A The total sales repeated on every row

B A running total by date

C An error

D Only the first row's sales

Without ORDER BY, the sum is over the entire partition, so every row shows the same total.

Which of these is a correct use of SUM() OVER() to get a running total?

A SELECT SUM(sales) FROM sales_table;

B SELECT sales, SUM(sales) OVER (ORDER BY date) FROM sales_table;

C SELECT sales, SUM(sales) FROM sales_table GROUP BY date;

D SELECT sales, SUM(sales) OVER () FROM sales_table ORDER BY date;

Option B correctly uses SUM() OVER (ORDER BY date) to calculate a running total.

What is the main benefit of using SUM() OVER() for running totals?

A It calculates totals without losing individual row details

B It filters rows based on sum

C It reduces the number of rows returned

D It sorts the table permanently

Using SUM() OVER() keeps all rows and adds a running total column, preserving detail.

Explain how to calculate a running total of sales by date using SQL window functions.

Think about how to add a cumulative sum column without grouping rows.

Describe the difference between using SUM() with GROUP BY and SUM() OVER() for totals.

Consider how the number of rows changes and what totals represent.

Practice

1. What does the SQL expression SUM(amount) OVER (ORDER BY date) calculate?

easy

A. A running total of the amount column ordered by date

B. The total sum of amount for all rows without order

C. The average amount grouped by date

D. The maximum amount value in the table

Solution

1. Step 1: Understand SUM() with OVER()

   The SUM() function adds values, and OVER() defines how to apply it across rows.

2. Step 2: Analyze ORDER BY in OVER()

   ORDER BY date means the sum accumulates in date order, creating a running total.

3. Final Answer:

   A running total of the amount column ordered by date -> Option A

4. Quick Check:

   SUM() OVER(ORDER BY) = running total [OK]

Hint: SUM() with ORDER BY inside OVER() means running total [OK]

Common Mistakes:

• Thinking SUM() without OVER() gives running total
• Confusing running total with group total
• Ignoring ORDER BY inside OVER()

2. Which of the following is the correct syntax to calculate a running total of sales ordered by sale_date?

easy

A. SELECT sales, SUM(sales) OVER (PARTITION BY sale_date) FROM sales_table;

B. SELECT sales, SUM(sales) OVER (ORDER BY sale_date) FROM sales_table;

C. SELECT sales, SUM(sales) FROM sales_table ORDER BY sale_date;

D. SELECT sales, SUM(sales) OVER () ORDER BY sale_date FROM sales_table;

Solution

1. Step 1: Identify correct use of SUM() OVER()

   Running total requires SUM() OVER with ORDER BY clause inside OVER().

2. Step 2: Check each option's syntax

   SELECT sales, SUM(sales) OVER (ORDER BY sale_date) FROM sales_table; uses SUM(sales) OVER (ORDER BY sale_date), which is correct syntax for running total.

3. Final Answer:

   SELECT sales, SUM(sales) OVER (ORDER BY sale_date) FROM sales_table; -> Option B

4. Quick Check:

   SUM() OVER (ORDER BY ...) = running total syntax [OK]

Hint: Use SUM() OVER (ORDER BY column) for running totals [OK]

Common Mistakes:

• Using PARTITION BY instead of ORDER BY for running total
• Omitting OVER() clause
• Placing ORDER BY outside OVER()

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

SELECT id, sale_date, amount, SUM(amount) OVER (ORDER BY sale_date) AS running_total FROM sales_data ORDER BY id;

Assume the data:
id | sale_date | amount
1 | 2024-01-01 | 100
2 | 2024-01-03 | 200
3 | 2024-01-02 | 150

medium

A. Running totals: 100, 350, 450 ordered by id

B. Running totals: 100, 150, 350 ordered by id

C. Running totals: 100, 450, 250 ordered by id

D. Running totals: 100, 300, 450 ordered by id

Solution

1. Step 1: Order rows by sale_date for running total

   Order by sale_date: id=1(2024-01-01), id=3(2024-01-02), id=2(2024-01-03).

2. Step 2: Calculate running total in sale_date order

   Running totals: 100 (id=1), 100+150=250 (id=3), 250+200=450 (id=2).

3. Step 3: Output ordered by id

   Order by id: id=1 (100), id=2 (450), id=3 (250).

4. Final Answer:

   Running totals: 100, 450, 250 ordered by id -> Option C

5. Quick Check:

   Running total sums in sale_date order, output sorted by id [OK]

Hint: Running total sums by ORDER BY inside OVER, output order separate [OK]

Common Mistakes:

• Assuming running total follows output ORDER BY
• Adding amounts in id order instead of sale_date order
• Confusing running total with simple sum

4. Identify the error in this query intended to calculate a running total of points ordered by game_date:

SELECT player_id, points, SUM(points) OVER (PARTITION BY player_id) ORDER BY game_date FROM game_scores;

medium

A. ORDER BY clause is outside the OVER() function

B. PARTITION BY cannot be used with SUM()

C. Missing GROUP BY clause

D. SUM() cannot be used with window functions

Solution

1. Step 1: Check placement of ORDER BY

   ORDER BY for running total must be inside OVER(), not outside the function.

2. Step 2: Understand correct syntax

   Correct syntax: SUM(points) OVER (PARTITION BY player_id ORDER BY game_date).

3. Final Answer:

   ORDER BY clause is outside the OVER() function -> Option A

4. Quick Check:

   ORDER BY must be inside OVER() for running totals [OK]

Hint: Put ORDER BY inside OVER() for running totals [OK]

Common Mistakes:

• Placing ORDER BY outside OVER()
• Confusing PARTITION BY with GROUP BY
• Thinking SUM() can't be window function

5. You have a sales table with columns region, sale_date, and amount. You want to calculate a running total of sales per region ordered by sale_date. Which query correctly achieves this?

hard

A. SELECT region, sale_date, amount, SUM(amount) OVER (ORDER BY sale_date) AS running_total FROM sales;

B. SELECT region, sale_date, amount, SUM(amount) OVER (PARTITION BY sale_date ORDER BY region) AS running_total FROM sales;

C. SELECT region, sale_date, amount, SUM(amount) FROM sales GROUP BY region, sale_date ORDER BY sale_date;

D. SELECT region, sale_date, amount, SUM(amount) OVER (PARTITION BY region ORDER BY sale_date) AS running_total FROM sales;

Solution

1. Step 1: Understand requirement for running total per region

   We need to reset running total for each region, so PARTITION BY region is required.

2. Step 2: Check ORDER BY for running total

   ORDER BY sale_date inside OVER() ensures running total accumulates by date within each region.

3. Step 3: Evaluate options

   SELECT region, sale_date, amount, SUM(amount) OVER (PARTITION BY region ORDER BY sale_date) AS running_total FROM sales; correctly uses PARTITION BY region and ORDER BY sale_date inside SUM() OVER().

4. Final Answer:

   SELECT region, sale_date, amount, SUM(amount) OVER (PARTITION BY region ORDER BY sale_date) AS running_total FROM sales; -> Option D

5. Quick Check:

   PARTITION BY region + ORDER BY sale_date = running total per region [OK]

Hint: Use PARTITION BY for groups, ORDER BY for running total order [OK]

Common Mistakes:

• Omitting PARTITION BY to separate groups
• Using GROUP BY instead of window functions
• Swapping ORDER BY columns inside OVER()