Joining With Null Handling | Dataford Interview Questions

How would you handle missing values and nulls when joining two large user behavior datasets?

Write a PostgreSQL query using the two provided datasets. Exclude records without a usable user ID or activity date, preserve valid user-date combinations found in either dataset, and treat missing behavior counts as zero.

Output

One row per valid user and activity date found in either dataset
Columns: user_id, activity_date, dataset_a_sessions, dataset_b_sessions, total_sessions, and coverage_status
Order by activity_date ascending, then user_id ascending

Schema

user_behavior_a

Column	Type	Description
event_idPK	INT	Unique event identifier
user_id	INT	Identifier for the user associated with the event
activity_date	DATE	Date on which the behavior occurred
session_count	INT	Number of sessions recorded by the event

user_behavior_b

Column	Type	Description
event_idPK	INT	Unique event identifier
user_id	INT	Identifier for the user associated with the event
activity_date	DATE	Date on which the behavior occurred
session_count	INT	Number of sessions recorded by the event

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Problem