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#0343

Union Active Users From Two Sources

EasySQL15 min10 XP

Problem

Return distinct user_id values appearing in web_events or app_events.

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

web_events(user_id INTEGER); app_events(user_id INTEGER)

Examples

Example 1
Inputuser 2 appears in both tables
Output1, 2, 3

UNION removes duplicates across both result sets.

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
Inputuser 2 appears in both tables
Expected[[1],[2],[3]]

UNION removes duplicates across both result sets.

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