Why does the running total calculated by SUM(amount) OVER (ORDER BY date) sometimes produce unexpected results when there are duplicate dates?

hard📝 Conceptual Q10 of 15

SQL - Advanced Window Functions

Why does the running total calculated by SUM(amount) OVER (ORDER BY date) sometimes produce unexpected results when there are duplicate dates?

ABecause SUM() cannot handle duplicate values

BBecause rows with the same date have no defined order, causing inconsistent running totals

CBecause ORDER BY date sorts rows randomly

DBecause window functions ignore duplicate rows

Step-by-Step Solution

Solution:

Step 1: Understand ordering in window functions
ORDER BY date orders rows, but if dates are duplicated, order among those rows is undefined.
Step 2: Effect on running total
Undefined order causes running total to vary between executions or produce unexpected sums.
Final Answer:
Because rows with the same date have no defined order, causing inconsistent running totals -> Option B
Quick Check:
Duplicate ORDER BY values cause undefined row order [OK]

Quick Trick: Add unique column to ORDER BY to fix running total with duplicates [OK]

Common Mistakes:

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