📌 Question 88: Find Customers Who Purchased in Every Month of a Year

Table: orders (customer_id, order_date)

SELECT

    customer_id

FROM orders

WHERE EXTRACT(YEAR FROM order_date) = 2025

GROUP BY customer_id

HAVING COUNT(

    DISTINCT EXTRACT(MONTH FROM order_date)

) = 12;

📌 Question 89: Find the Most Frequently Bought Product After Product A

Table: order_items (order_id, product_id, sequence_no)

SELECT

    b.product_id,

    COUNT(*) AS purchase_count

FROM order_items a

JOIN order_items b

ON a.order_id = b.order_id

AND b.sequence_no = a.sequence_no + 1

WHERE a.product_id = 'Product_A'

GROUP BY b.product_id

ORDER BY purchase_count DESC

LIMIT 1;

📌 Question 90: Calculate Customer Lifetime in Days

Tables: customers (customer_id) orders (customer_id, order_date)

SELECT

    customer_id,

    MAX(order_date) - MIN(order_date) AS lifetime_days

FROM orders

GROUP BY customer_id;

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