[Solved] Given the table sales with columns month and revenue, what will this query return?SELECT month, revenue, LAG(revenue) OVER (ORDER BY month) AS prev_revenue FROM sales; | PostgreSQL

PostgreSQL - Window Functions in PostgreSQL

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

SELECT month, revenue, LAG(revenue) OVER (ORDER BY month) AS prev_revenue FROM sales;

AEach row shows only current month revenue

BEach row shows current month revenue and next month's revenue

CEach row shows current month revenue and previous month's revenue

DQuery will cause a syntax error

Step-by-Step Solution

Solution:

Step 1: Understand LAG with ORDER BY month
The LAG(revenue) returns the revenue from the previous month ordered by month.
Step 2: Interpret query output
Each row shows current month revenue and previous month's revenue in prev_revenue. The first row's prev_revenue will be NULL.
Final Answer:
Each row shows current month revenue and previous month's revenue -> Option C
Quick Check:
LAG = previous row value [OK]

Quick Trick: LAG returns previous row's value based on ORDER BY [OK]

Common Mistakes:

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