Getting hash map result from sql query [duplicate]

I want to get a data structure like this after the sql query:-Users table has username and city. It has 10000 records

    {"cities_arry": {"NY": ["john", "Mich", "Roh", "Dh", "Vir"],"KL": ["Big", "ching", "qull"],          ...     }        }

update: it is not possible to store nested hash on redis. So have to use MongoDb or some other tool.

• Suppose you have a table users and you have city and username, and you want to find a data structure like above, then how would you approach

How to get faster query result to get data structure like this.