Replace Employee ID with Unique Identifier

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Medium

Description

You have two tables: Employees and EmployeeUNI. Each employee has an id and a name. Some employees have been assigned a unique_id in the EmployeeUNI table, but not all. Write a SQL query to show the unique_id and name of each employee. If an employee does not have a unique_id, show NULL instead. Return the result in any order. (id, unique_id) is the primary key of EmployeeUNI.

Database Schema

Employees

Column Name	Type	Description
id	INT	Primary key
name	VARCHAR	Employee name

EmployeeUNI

Column Name	Type	Description
id	INT	FK to Employees
unique_id	INT	Unique identifier assigned

Example

Employees

id	name
1	Alice
7	Bob
11	Meir
90	Winston
3	Jonathan

EmployeeUNI

id	unique_id
3	1
11	2
90	3

Output

unique_id	name
NULL	Alice
NULL	Bob
2	Meir
3	Winston
1	Jonathan

Explanation:

Alice and Bob have no unique_id - show NULL. Meir → 2, Winston → 3, Jonathan → 1.

Approach hint

Start with a simple approach, explain the trade-off, then move toward a cleaner or more scalable solution.

Common mistake

Skipping assumptions, edge cases, or trade-offs can make an otherwise good answer feel incomplete.