# Replace Employee ID With The Unique Identifier Problem

## Description

LeetCode Problem 1378.

Table: Employees

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+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| name          | varchar |
+---------------+---------+
id is the primary key for this table.
Each row of this table contains the id and the name of an employee in a company.
``````

Table: EmployeeUNI

``````1
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+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| unique_id     | int     |
+---------------+---------+
(id, unique_id) is the primary key for this table.
Each row of this table contains the id and the corresponding unique id of an employee in the company.
``````

Write an SQL query to show the unique ID of each user. If a user doesn’t have a unique ID replace just show null.

Return the result table in any order.

The query result format is in the following example:

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Employees table:
+----+----------+
| id | name     |
+----+----------+
| 1  | Alice    |
| 7  | Bob      |
| 11 | Meir     |
| 90 | Winston  |
| 3  | Jonathan |
+----+----------+

EmployeeUNI table:
+----+-----------+
| id | unique_id |
+----+-----------+
| 3  | 1         |
| 11 | 2         |
| 90 | 3         |
+----+-----------+

EmployeeUNI table:
+-----------+----------+
| unique_id | name     |
+-----------+----------+
| null      | Alice    |
| null      | Bob      |
| 2         | Meir     |
| 3         | Winston  |
| 1         | Jonathan |
+-----------+----------+

Alice and Bob don't have a unique ID, We will show null instead.
The unique ID of Meir is 2.
The unique ID of Winston is 3.
The unique ID of Jonathan is 1.
``````

## MySQL Solution

``````1
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select unique_id, name
from Employees
left join EmployeeUNI
on if (Employees.id = EmployeeUNI.id, Employees.id, null)
``````