SQL1378.sql

-- Table: Employees

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

-- Table: EmployeeUNI

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

-- Write a solution to show the unique ID of each user, If a user does not have a unique ID replace just show null.

-- Return the result table in any order.

-- The result format is in the following example.

 

-- Example 1:

-- Input: 
-- 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         |
-- +----+-----------+
-- Output: 
-- +-----------+----------+
-- | unique_id | name     |
-- +-----------+----------+
-- | null      | Alice    |
-- | null      | Bob      |
-- | 2         | Meir     |
-- | 3         | Winston  |
-- | 1         | Jonathan |
-- +-----------+----------+
-- Explanation: 
-- Alice and Bob do not 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.


-- Solution (PostgreSQL):
select case 
       when EmployeeUNI.unique_id is null then null
       else EmployeeUNI.unique_id
       end as unique_id,
       Employees.name
from Employees 
     left join EmployeeUNI on (Employees.id = EmployeeUNI.id);