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ML Academy

SQL and Python

Interview question

  • midlevel
  • 8 minsuggested time
  • 3Data analyst, Data scientist, ML engineer

Two queries in a code review. (1) A report meant to list every flight-day with its web promotion, if any, uses flight_day f LEFT JOIN promotions p ON ... WHERE p.channel = 'web', and flights without promotions are missing. (2) A query for flights that never had a promotion uses WHERE f.flight_no NOT IN (SELECT flight_no FROM promotions) and returns zero rows, although such flights exist. Explain both bugs and fix them.

Tests: SQL NULL semantics and join filtering: where a predicate belongs (ON vs WHERE) in outer joins, three-valued logic in NOT IN, and anti-join patterns.

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