SQL First Message Per User | Dataford Interview Questions

Tinder's messaging analytics team wants to identify the first message sent by each user. This helps measure how quickly new matches begin conversations.

Write a PostgreSQL query against user_messages to return the first message for every user with at least one non-null timestamp.

Requirements

Use a window function to rank messages separately for each user_id.
Select the earliest non-null timestamp for each user.
If a user has multiple messages at the same timestamp, choose the lexicographically smallest message_id as the deterministic tie-breaker.
Return user_id, message_id, and timestamp, ordered by user_id.

Schema

user_messages

Column	Type	Description
user_id	INTEGER	Tinder user who sent the message
message_idPK	VARCHAR(12)	Unique identifier for the message
timestamp	TIMESTAMPTZ	Timestamp when the message was sent

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