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#0338
Seven Day Retention
MediumSQL24 min20 XP
Problem
Return user_id for users with an event exactly 7 days after signup.
Why This Matters
SQL interviews usually test whether you can turn a business question into the right rows, grouping level, join type, and edge-case behavior.
Schema
users(user_id INTEGER, signup_date TEXT); events(user_id INTEGER, event_date TEXT)
Examples
Example 1
Inputusers 1 and 3 return exactly seven days later
Output1 and 3
DATE(signup_date, +7 day) computes the target retention date.
Constraints
- Return only the requested columns.
- Use deterministic ORDER BY when multiple rows are returned.
- Assume SQLite-compatible SQL unless the prompt states otherwise.
CodeSQL
Visible browser tests run here when available.
Testcases1 visible / 3 hidden categories
Core SQL case
Inputusers 1 and 3 return exactly seven days later
Expected[[1],[3]]
DATE(signup_date, +7 day) computes the target retention date.
Hidden Test Categories
Empty or missing matchesDuplicate rows or tiesNULL values where the pattern commonly breaks