My solution to the BST challenge

bst1.sql

-- https://www.hackerrank.com/challenges/binary-search-tree-1

select node, type from (
    (select N as node, "Root" as type from BST where P is null)
    union
    (select t1.N as node, "Leaf" as type from BST t1
        left join BST t2 on t1.N = t2.P
        where t2.P is null)
    union
    (select distinct t1.N as node, "Inner" as type from BST t1
        left join BST t2 on t1.N = t2.P
        where t1.P is not null and t2.P is not null)
) as U order by node;