You want to find the cumulative count of orders per customer ordered by order date. Which query correctly uses COUNT as a window function to achieve this?

hard📝 Application Q15 of 15

PostgreSQL - Window Functions in PostgreSQL

ASELECT customer_id, order_date, COUNT(*) FROM orders GROUP BY customer_id, order_date ORDER BY order_date;

BSELECT customer_id, order_date, COUNT(*) OVER (ORDER BY order_date) AS cumulative_orders FROM orders;

CSELECT customer_id, order_date, COUNT(*) OVER (PARTITION BY customer_id ORDER BY order_date) AS cumulative_orders FROM orders;

DSELECT customer_id, order_date, COUNT(*) OVER (PARTITION BY order_date ORDER BY customer_id) AS cumulative_orders FROM orders;

Step-by-Step Solution

Solution:

Step 1: Understand cumulative count per customer
COUNT(*) OVER (PARTITION BY customer_id ORDER BY order_date) counts rows per customer in order of order_date cumulatively.
Step 2: Check other options for correctness
SELECT customer_id, order_date, COUNT(*) FROM orders GROUP BY customer_id, order_date ORDER BY order_date; uses GROUP BY, not cumulative count; C lacks partition by customer; D partitions by order_date, not customer.
Final Answer:
SELECT customer_id, order_date, COUNT(*) OVER (PARTITION BY customer_id ORDER BY order_date) AS cumulative_orders FROM orders; -> Option C
Quick Check:
Use PARTITION BY customer_id with ORDER BY for cumulative count [OK]

Quick Trick: Partition by customer and order by date for cumulative count [OK]

Common Mistakes:

Missing PARTITION BY customer_id
Using GROUP BY instead of window function
Partitioning by wrong column

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You want to find the cumulative count of orders per customer ordered by order date. Which query correctly uses COUNT as a window function to achieve this?

Step 1: Understand cumulative count per customer

Step 2: Check other options for correctness

Final Answer:

Quick Check:

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