Find Duplicate Rows in SQL | Dataford Interview Questions

Your question is Find Duplicate Rows in SQL. Start with the requirements and the two tables on the right.

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Problem

EPAM Systems uses several intake channels, including the EPAM Client Portal and EPAM CRM, to collect client contact records. Data quality analysts need to identify repeated contact values among records received from active source systems.

Write a PostgreSQL query that returns duplicate combinations of email and phone from active source systems.

Requirements

Join client_contacts to source_systems using source_system_id.
Include only source systems where is_active is true.
Group by email and phone, return only groups occurring more than once, and include the duplicate count.
Order by duplicate count descending, then email ascending, with NULL emails last and NULL phones first.

Schema

client_contacts

Column	Type	Description
contact_idPK	INT	Unique contact record identifier
email	VARCHAR(255)	Client email address
phone	VARCHAR(30)	Client phone number
source_system_id	INT	References the intake source system
created_at	DATE	Date the contact record was created

source_systems

Column	Type	Description
source_system_idPK	INT	Unique source system identifier
source_system_name	VARCHAR(100)	Name of the intake system
is_active	BOOLEAN	Whether the source system is active