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GROUP BY allows the database to collect all rows belonging to the same state into one group. After grouping, aggregation functions can combine the city names into a single result for each state.
Problems involving SQL grouping and aggregation are common in technical interviews, including FAANG-style assessments. While the exact problem may vary, understanding GROUP BY and string aggregation is frequently tested.
The optimal approach is to group rows by state using the SQL GROUP BY clause and aggregate the city names with functions like STRING_AGG or GROUP_CONCAT. Sorting the cities inside the aggregation ensures the output is consistent and readable.
Functions such as STRING_AGG (PostgreSQL/SQL Server) or GROUP_CONCAT (MySQL) are commonly used. These functions combine multiple row values into a single string while optionally allowing ordering of the values.