Build a Month-by-Month Cohort Retention SQL Report

Write a SQL cohort retention analysis using the table `{{table_name}}` in `{{database_type}}`.

Definitions:
- Cohort: the month of a user's first `{{cohort_event}}` recorded in `{{date_column}}`
- Retention: whether the user performed `{{retention_event}}` in each subsequent month

The query should:
1. Define cohorts using a CTE that finds each user's first event month
2. Join back to activity data to find which months each user was active
3. Calculate cohort size and retention count per month offset (0, 1, 2, ... N)
4. Return a cohort × month offset matrix with retention percentages

Include comments on each CTE. Database: `{{database_type}}`.