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SQL Interview Series — Part 11

📌 Question 11: Calculate a Running Total of Sales

Suppose you have a Sales table:

sale_id | sale_date  | amount
--------|------------|-------
1       | 2026-01-01 | 1000
2       | 2026-01-02 | 1500
3       | 2026-01-03 | 800
4       | 2026-01-04 | 1200

❓ Calculate the cumulative (running) sales amount for each day.

💡 Approach

A running total means:

Current day's sales + all previous sales

For example:

1000

1000 + 1500 = 2500

2500 + 800 = 3300

3300 + 1200 = 4500

The easiest approach is to use the "SUM()" window function.

📌 SQL Solution

SELECT
    sale_date,
    amount,
    SUM(amount) OVER (
        ORDER BY sale_date
        ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
    ) AS running_total
FROM Sales
ORDER BY sale_date;

🔎 How does it work?

1️⃣ "ORDER BY sale_date"

Determines the order in which the running total is calculated.

2️⃣ "SUM(amount) OVER(...)"

Calculates the sum without collapsing the rows like a normal "GROUP BY".

3️⃣ "UNBOUNDED PRECEDING"

Means:

Start from the first row.

4️⃣ "CURRENT ROW"

Means:

Include the current row in the total.

So the calculation becomes:

Date        Amount    Running Total
----------  --------  -------------
Jan 01       1000        1000
Jan 02       1500        2500
Jan 03        800        3300
Jan 04       1200        4500

⚠️ Interview Trap

Don't use:

GROUP BY sale_date

for a running total.

"GROUP BY" collapses rows into groups.

A window function allows us to calculate an aggregate while keeping individual rows.

Also, if multiple rows can have the same "sale_date", be careful about the ordering. A unique column such as "sale_id" may need to be included:

SUM(amount) OVER (
    ORDER BY sale_date, sale_id
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
)

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