Running totals with SUM OVER in SQL - Step-by-Step Execution

Concept Flow - Running totals with SUM OVER

Start with table rows

↓

Order rows by a column

↓

For each row, calculate SUM of values from first row to current row

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Add running total as new column

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Return result with running totals

The query orders rows and calculates a running total by summing values from the start up to each row.

Execution Sample

SQL

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

Calculates running total of 'amount' ordered by 'id' in the sales table.

Execution Table

Step	Row id	amount	Running total calculation	running_total
1	1	100	SUM(amount) from id=1 to 1 = 100	100
2	2	200	SUM(amount) from id=1 to 2 = 100 + 200	300
3	3	150	SUM(amount) from id=1 to 3 = 100 + 200 + 150	450
4	4	50	SUM(amount) from id=1 to 4 = 450 + 50	500
5	5	300	SUM(amount) from id=1 to 5 = 500 + 300	800
6	End		No more rows to process

💡 All rows processed, running totals calculated for each row.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
running_total	0	100	300	450	500	800	800

Key Moments - 2 Insights

Why does the running total increase with each row?

Because SUM OVER adds the current row's amount to the sum of all previous rows, as shown in execution_table rows 1 to 5.

What happens if the ORDER BY clause is missing in SUM OVER?

Without ORDER BY, the running total is not calculated in a meaningful order, so results may be incorrect or unpredictable, unlike the ordered steps in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the running_total at step 3?

A 150

B 300

C 450

D 500

💡 Hint

Check the 'running_total' column at step 3 in the execution_table.

At which step does the running_total first exceed 400?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the running_total values in execution_table rows to find when it passes 400.

If the amount in row with id=4 was 100 instead of 50, what would be the running_total at step 4?

A 600

B 550

C 500

D 450

💡 Hint

Add the new amount 100 to the running_total at step 3 (450) to find the new total.

Concept Snapshot

SUM() OVER (ORDER BY column) calculates a running total.
It sums values from the first row up to the current row.
ORDER BY defines the order of rows for summing.
Running totals help track cumulative sums over ordered data.

Full Transcript

This visual execution shows how the SQL query calculates running totals using SUM OVER. The query orders rows by id and sums the amount column from the first row up to each current row. The execution table traces each step, showing how the running total grows as rows are processed. The variable tracker highlights the running_total value after each row. Key moments clarify why ordering is important and how the running total accumulates. The quiz tests understanding of running total values at specific steps and effects of changing data. This helps beginners see how running totals work step-by-step in SQL.

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()