Common Users Between Two Tables | Dataford Interview Questions

Paytm wants to identify users who used both Paytm Wallet and Paytm UPI during January 2026. Write a PostgreSQL query that returns the common user_id values from the two activity tables.

Requirements

Consider only activity from 2026-01-01 through 2026-01-31, inclusive.
Return each user only once, even if the user has multiple activity records in either table.
Exclude rows where user_id is NULL.
Sort the result by user_id in ascending order.

Schema

wallet_activity

Column	Type	Description
event_idPK	INT	Unique Paytm Wallet activity event
user_id	INT	Paytm user identifier
activity_date	DATE	Date of Wallet activity

upi_activity

Column	Type	Description
event_idPK	INT	Unique Paytm UPI activity event
user_id	INT	Paytm user identifier
activity_date	DATE	Date of UPI activity

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