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

Concept Flow - Running totals with SUM OVER

Start with table rows

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Order rows by a column

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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.