Running Total in SQL

Running Total in SQL (Beginner-Friendly Step by Step)

One of the most useful and beginner-friendly SQL window function problems is calculating a
👉 Running Total (Cumulative Sum).

If you understand this, you unlock analytics, finance, and reporting queries.

Let’s learn it slowly and clearly 👇

🧠 What is a Running Total?

A running total means:

Add the current value to all previous values.

Example numbers:

100 → 150 → 200

Running total:

100 → 250 → 450

📊 Sample Data

❌ Wrong Way (Beginner Mistake)

SELECT SUM(amount)
FROM sales;

This gives:

450

❌ You lose row-level details
❌ Not a running total

✅ Correct Way: Use a Window Function

Step 1: Use SUM() as a window function

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

🔍 How SQL Thinks (Very Important)

1. Sort rows by day
2. Start adding amounts from the first row
3. Keep adding as rows increase

✅ Final Output

✔ Each row keeps its value
✔ Total keeps growing

🧠 Key Keywords Explained Simply

ORDER BY

👉 Tells SQL which order to add values

OVER()

👉 Tells SQL don’t collapse rows

📌 Running Total Per Group (Very Common)

Example:

Running total per product

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

✔ Restarts total for each product

🎯 Where Running Totals Are Used

• Bank balance over time
• Daily revenue tracking
• Monthly sales growth
• Inventory movement
• Expense accumulation

Simple Rule to Remember 🧠

SUM() + OVER() + ORDER BY = Running Total

💬 Interview Question for You

How would you calculate a running total that resets every month?

(Hint: PARTITION BY month 😉)