COALESCE in SQL | Dataford Interview Questions

What is the Coalesce statement in Sql?

Asked in the Technical interview stage. Write a PostgreSQL query that returns the first usable contact value for every employee, using the employee email when no usable preference exists.

Output

One row per employee.
Columns: employee_id, employee_name, and contact_email.
Replace missing or empty preferred values with the employee email, then Not provided when neither value exists.
Order by employee_id ascending.

Schema

employees

Column	Type	Description
employee_idPK	INT	Unique employee identifier
employee_name	VARCHAR(100)	Employee display name
email	VARCHAR(255)	Default employee email address

contact_preferences

Column	Type	Description
preference_idPK	INT	Unique contact preference identifier
employee_id	INT	Employee associated with the preference
preferred_email	VARCHAR(255)	Preferred email address, if supplied

Your question is COALESCE in SQL. Start with the requirements and the two tables in the Question tab.

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Problem