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#0393
Pairs of Users in Same Country
MediumSQL24 min20 XP
Problem
Return user_id_1 and user_id_2 pairs where both users are in the same country. Return each pair once.
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, country TEXT)
Examples
Example 1
Inputusers 1, 2, and 4 are in IN
Outputthree unique pairs
The less-than condition avoids duplicate reversed pairs and self-pairs.
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, 2, and 4 are in IN
Expected[[1,2],[1,4],[2,4]]
The less-than condition avoids duplicate reversed pairs and self-pairs.
Hidden Test Categories
Empty or missing matchesDuplicate rows or tiesNULL values where the pattern commonly breaks