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#0368

Daily New Users by First Event

MediumSQL24 min20 XP

Problem

Return first_event_date and new_users_count.

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

events(user_id INTEGER, event_date TEXT)

Examples

Example 1
Inputusers 1 and 2 first appear Jan 1
OutputJan 1 -> 2

First compute one first date per user, then count users per 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 2 first appear Jan 1
Expected[["2026-01-01",2],["2026-01-03",1]]

First compute one first date per user, then count users per date.

Hidden Test Categories
Empty or missing matchesDuplicate rows or tiesNULL values where the pattern commonly breaks