[HackerRank] Binary Three Nodes_CASE WHEN IN(SUBQUERY)

문제

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.

Write a query to find the node type of Binary Tree ordered by the value of the node. Output one of the following for each node:

• Root: If node is root node.
• Leaf: If node is leaf node.
• Inner: If node is neither root nor leaf node.

Sample Input

Sample Output

1 Leaf
2 Inner
3 Leaf
5 Root
6 Leaf
8 Inner
9 Leaf

Explanation

The Binary Tree below illustrates the sample:

 

풀이

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

 

배운 점

Parent가 Null = Root
P table 안에 N이 있다 = Inner
이도 저도 아님 = Leaf

CASE WHEN THEN ELSE END AS ___