How would you write a query to compute a running total of sales per customer, while still displaying each individual sale record?

hard📝 Application Q9 of 15

SQL - Window Functions Fundamentals

ASELECT customer_id, sale_amount, SUM(sale_amount) OVER (PARTITION BY customer_id ORDER BY sale_date) AS running_total FROM sales;

BSELECT customer_id, SUM(sale_amount) FROM sales GROUP BY customer_id ORDER BY sale_date;

CSELECT customer_id, sale_amount, SUM(sale_amount) FROM sales GROUP BY customer_id, sale_date;

DSELECT customer_id, sale_amount, SUM(sale_amount) FROM sales;

Step-by-Step Solution

Solution:

Step 1: Understand running total requirement
Calculate cumulative sum per customer ordered by sale date, showing each sale.
Step 2: Analyze options
SELECT customer_id, sale_amount, SUM(sale_amount) OVER (PARTITION BY customer_id ORDER BY sale_date) AS running_total FROM sales; uses a window function with PARTITION BY customer_id and ORDER BY sale_date, which computes running total per customer without collapsing rows.
Step 3: Why others are incorrect
Options B, C, and D use GROUP BY or no window function, which aggregate rows and lose individual sale details.
Final Answer:
SELECT customer_id, sale_amount, SUM(sale_amount) OVER (PARTITION BY customer_id ORDER BY sale_date) AS running_total FROM sales; correctly computes running total while preserving rows.
Quick Check:
Window functions with ORDER BY enable running totals. [OK]

Quick Trick: Use window SUM() OVER with ORDER BY for running totals. [OK]

Common Mistakes:

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