SQL Challenges

query.sql

CREATE TABLE IF NOT EXISTS USERS (
   ID serial primary key not null,
   UserName varchar(200)null,
   Parent int not null
);

INSERT INTO USERS (UserName, Parent) VALUES ('Ali', 2)
INSERT INTO USERS (UserName, Parent) VALUES ('Budi', 0)
INSERT INTO USERS (UserName, Parent) VALUES ('Cecep', 1)

CREATE INDEX idx_id_user 
ON USERS(ID);

EXPLAIN
SELECT u.ID, u.UserName, 
	CASE 
		WHEN us.UserName is NULL THEN '' 
		ELSE us.UserName 
		END ParentUserName
FROM USERS u 
LEFT JOIN USERS us 
ON u.Parent = us.ID
ORDER BY u.ID ASC