Why should the GROUP BY clause include the distance column instead of destination in this query?

In the original query, the GROUP BY clause grouped by the composite key of src_dest and dest_src, which aggregates all routes between two cities regardless of distance. This causes the MIN(distance) to be computed across all routes, potentially returning the shortest distance for a different direction or a different pair of cities. By adding distance to the GROUP BY, each unique combination of src_dest, dest_src, and distance becomes a distinct group. This ensures that MIN(distance) is evaluated within each exact route, preserving the correct shortest path for each direction. The fix also aligns with SQL semantics: columns in the SELECT list that are not aggregated must appear in the GROUP BY. Without including distance, the query violates this rule in many SQL engines, leading to ambiguous results or errors. Therefore, grouping by distance guarantees accurate aggregation and compliance with SQL standards.