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#0383
Customers With Exactly One Order
EasySQL15 min10 XP
Problem
Return customer_id values with exactly one order.
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
orders(order_id INTEGER, customer_id INTEGER)
Examples
Example 1
Inputcustomers 2 and 3 have one order
Output2 and 3
HAVING COUNT(*) = 1 keeps groups with exactly one row.
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
Inputcustomers 2 and 3 have one order
Expected[[2],[3]]
HAVING COUNT(*) = 1 keeps groups with exactly one row.
Hidden Test Categories
Empty or missing matchesDuplicate rows or tiesNULL values where the pattern commonly breaks