Given the table sales with columns region, month, and revenue, what will this query return?

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PostgreSQL - Window Functions in PostgreSQL

Given the table sales with columns region, month, and revenue, what will this query return?

SELECT region, month, revenue, FIRST_VALUE(revenue) OVER (PARTITION BY region ORDER BY month) AS first_rev FROM sales ORDER BY region, month;

AThe total revenue summed for each region.

BThe revenue of the last month for each region repeated on every row of that region.

CThe revenue of the first month for each region repeated on every row of that region.

DThe revenue of the current month only.

Step-by-Step Solution

Solution:

Step 1: Analyze FIRST_VALUE with PARTITION and ORDER BY
FIRST_VALUE(revenue) OVER (PARTITION BY region ORDER BY month) returns the first revenue value per region ordered by month.
Step 2: Understand output per row
For each row in a region, the first month's revenue is repeated as first_rev.
Final Answer:
The revenue of the first month for each region repeated on every row of that region. -> Option C
Quick Check:
FIRST_VALUE repeats first row's value per partition [OK]

Quick Trick: FIRST_VALUE repeats first sorted value per group [OK]

Common Mistakes:

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