SQL for Top Trading Users | Dataford Interview Questions

OKX Business Analytics needs to identify the highest-volume users on the OKX trading platform. Write a PostgreSQL query that ranks all users by their total trading volume during the 30-day period ending today.

Requirements

Include every user, including users with no qualifying trades, with zero volume for users without activity.
Include only trades whose executed_at falls between CURRENT_DATE - INTERVAL '29 days' and the end of CURRENT_DATE.
Use a window function to identify the top 10% of users. For this exercise, divide users into ten volume-ordered deciles with NTILE(10) and return users in decile 1.
Return the user ID, username, 30-day trading volume, and decile, ordered by volume descending and user ID ascending.

Schema

users

Column	Type	Description
user_idPK	INTEGER	Unique OKX user identifier
username	VARCHAR(50)	User display name
country_code	VARCHAR(2)	User registration country code

trades

Column	Type	Description
trade_idPK	BIGINT	Unique trade identifier
user_id	INTEGER	User who executed the trade
executed_at	DATE	Trade execution date
usd_volume	NUMERIC(18,2)	Trade volume converted to USD

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