Find Duplicates With SQL | Dataford Interview Questions

Nagarro's customer onboarding platform receives customer records from multiple intake sources. Data quality checks must identify email addresses that appear more than once so they can be reviewed before downstream processing.

Write a PostgreSQL query to find duplicate non-null email addresses in the customer_onboarding table.

Requirements

Group records by email and return only email values appearing at least twice.
Return the duplicate email, the number of occurrences, and the earliest created_at value for that email.
Sort by occurrence count descending, then email ascending.

Schema

customer_onboarding

Column	Type	Description
record_idPK	INT	Unique onboarding record identifier
email	VARCHAR(255)	Customer email address
full_name	VARCHAR(100)	Customer's full name
source_system	VARCHAR(50)	System that submitted the record
created_at	DATE	Date the onboarding record was created

Your question is Find Duplicates With SQL. Start with the requirements and the one table in the Question tab.

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Problem

Write a PostgreSQL query to find duplicate non-null email addresses in the customer_onboarding table.

Requirements

Group records by email and return only email values appearing at least twice.
Return the duplicate email, the number of occurrences, and the earliest created_at value for that email.
Sort by occurrence count descending, then email ascending.

Schema

customer_onboarding

Column	Type	Description
record_idPK	INT	Unique onboarding record identifier
email	VARCHAR(255)	Customer email address
full_name	VARCHAR(100)	Customer's full name
source_system	VARCHAR(50)	System that submitted the record
created_at	DATE	Date the onboarding record was created

Your question is Find Duplicates With SQL. Start with the requirements and the one table in the Question tab.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.