[Solved] Given the table orders with columns order_id and amount, what will this query return?SELECT order_id, amount, LAG(amount) OVER (ORDER BY order_id) AS prev_amount FROM orders; | PostgreSQL

PostgreSQL - Window Functions in PostgreSQL

Given the table orders with columns order_id and amount, what will this query return?

SELECT order_id, amount, LAG(amount) OVER (ORDER BY order_id) AS prev_amount FROM orders;

AEach row shows the current amount and the amount from the next order_id

BThe query will cause a syntax error

CEach row shows only the current amount repeated twice

DEach row shows the current amount and the amount from the previous order_id

Step-by-Step Solution

Solution:

Step 1: Understand LAG usage
LAG(amount) OVER (ORDER BY order_id) returns the amount from the previous row ordered by order_id.
Step 2: Analyze the output
For each order_id, the query shows its amount and the amount from the previous order_id row. The first row's prev_amount will be NULL.
Final Answer:
Each row shows the current amount and the amount from the previous order_id -> Option D
Quick Check:
LAG returns previous row's value, so output matches Each row shows the current amount and the amount from the previous order_id [OK]

Quick Trick: LAG returns previous row's value in ORDER BY sequence [OK]

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Given the table orders with columns order_id and amount, what will this query return?