HackerRank MySQL - Binary Tree Nodes

You are given a table, BST, containing two columns: N and P, where N represents the value of a node in Binary Tree, and P is the parent of N.

SELECT N,
    CASE
        WHEN P IS NULL THEN 'Root'
        WHEN N IN (SELECT P FROM BST) THEN 'Inner'
        ELSE 'Leaf'
    END
FROM BST
ORDER BY N;