Oct 08, 2026 by Offer

TikTok Interview Questions 2027: Second Day Confirmation SQL

For this TikTok interview question — users who confirmed on day two but not day one — join signups to confirmations on user_id and filter where the date gap equals exactly 1. State your date-function assumption before writing, and mention deduplication if users can confirm multiple times.

TikTok Interview Questions: What the Second-Day Confirmation Problem Tests

This SQL problem is commonly reported by candidates in TikTok's data-oriented interviews because it tests precise date logic on joined tables — a daily reality in growth analytics, where activation and retention funnels are built exactly this way. Interviewers are testing whether you can translate "confirmed on the second day but not the first" into an unambiguous filter, handle the join correctly, and reason about date boundaries. Vague date math is the classic failure.

TikTok Interview Questions: How to Solve Step by Step

Clarify the grain. "I'll assume the emails table holds one row per signup with a signup date, and the texts table holds confirmation actions with action dates. Let me confirm: can a user confirm multiple times?" Asking this first shows seniority.
Join on user. INNER JOIN emails to texts on user_id — you only care about users present in both.
Compute the day gap. Use the dialect's date difference (DATEDIFF in MySQL/SQL Server, DATE_DIFF in BigQuery — name your assumption): keep rows where the gap between signup date and confirmation date equals exactly 1.
Exclude same-day confirmers. The = 1 condition naturally excludes day-zero confirmations — but say this explicitly, since "did not confirm on the first day" is part of the spec.
Deduplicate. If multiple confirmations per user are possible, use SELECT DISTINCT user_id or GROUP BY user_id — mention why.
Sanity-check the logic. Narrate one example: "A user signing up Monday and confirming Tuesday has gap 1 — included; confirming Monday has gap 0 — excluded."

Example line: "The whole problem reduces to a join plus one precise date filter — the skill being tested is turning 'second day' into an exact, defensible predicate."

Common Mistakes

Off-by-one on "second day." Treating it as gap = 2 instead of gap = 1 — define day one as the signup day explicitly.
Ignoring duplicate confirmations. Without DISTINCT, users with multiple texts get double-counted.
Assuming the date function. DATEDIFF argument order and units differ by dialect — state your assumption.

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FAQ

Which SQL dialect should I use? The one you know best — just name it and its date functions upfront.

Should I handle time zones? Mention it as an edge case ("if timestamps cross midnight in different zones...") without derailing the core solution.

What if a user confirms on both day 1 and day 2? Per the spec they confirmed on day one, so exclude them — add a NOT EXISTS or HAVING clause and explain the choice.

How do I show I understand the business context? One line: this is a next-day activation metric, the kind of funnel analysis growth teams run constantly.

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