🔥 FAANG SQL Interview Question

📊 Find users who placed orders on their first login day (same-day conversion)

Table: Logins

user_id | login_date

Table: Orders

user_id | order_date

💡 Query:

WITH first_login AS (
SELECT user_id,
MIN(login_date) AS first_login_date
FROM Logins
GROUP BY user_id
)
SELECT f.user_id
FROM first_login f
JOIN Orders o
ON f.user_id = o.user_id
AND f.first_login_date = o.order_date;

🎯 Why this matters:

✅ Tests multi-table joins + cohort logic
✅ Evaluates ability to derive first-event behavior
✅ Common in product analytics & conversion funnels

⚡ Pro Tip:

✅ Always isolate “first event” using MIN() in a CTE
✅ Join carefully on both user_id + date to avoid false matches

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