Back to Practice
#0374
Retention Rate by Cohort
HardSQL35 min25 XP
Problem
Return cohort_month and day_1_retention_rate for each signup month. Day 1 retained means an event exactly one day 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
InputJan cohort has 2 users and 1 retained
OutputJanuary rate 0.5
The numerator counts retained users; the denominator counts cohort users.
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
InputJan cohort has 2 users and 1 retained
Expected[["2026-01",0.5],["2026-02",1]]
The numerator counts retained users; the denominator counts cohort users.
Hidden Test Categories
Empty or missing matchesDuplicate rows or tiesNULL values where the pattern commonly breaks