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#0354

Avoid NOT IN With NULL Trap

MediumSQL24 min20 XP

Problem

Return customer_id values from customers that are not present in blocked_customers, even if blocked_customers contains NULL.

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

customers(customer_id INTEGER); blocked_customers(customer_id INTEGER)

Examples

Example 1
Inputblocked list contains 2 and NULL
Output1 and 3

NOT EXISTS is not poisoned by NULL values in the blocked table.

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
Inputblocked list contains 2 and NULL
Expected[[1],[3]]

NOT EXISTS is not poisoned by NULL values in the blocked table.

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