I knew roughly what they wanted but blanked on the exact syntax for LEAST/GREATEST for a second.
Normalize each route by ordering the city pair so that the smaller city comes first (e.g., using LEAST and GREATEST). Then select distinct normalized pairs. This ensures (A, B) and (B, A) are treated as the same route.
Pro tip: Mention that this approach is efficient because it avoids self-joins and works well with indexing. Also, clarify that the output should contain each unique route only once, regardless of direction.
Recognize that the goal is to return unique undirected city pairs from directed flight data. Each row represents a directed flight, but we need to treat (A, B) and (B, A) as the same.
For each row, create a normalized pair where the first city is the lexicographically smaller one. Use functions like LEAST and GREATEST (or CASE WHEN) to achieve this.
Apply DISTINCT to the normalized pairs to eliminate duplicates. This yields each unique route exactly once.
Combine the normalization and distinct selection into a single query. For example: SELECT DISTINCT LEAST(departure, arrival) AS city1, GREATEST(departure, arrival) AS city2 FROM flights;
Discuss handling of NULLs, case sensitivity, and whether the output should be ordered. Also, mention that if the table is large, indexing on the normalized columns could help.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.