LeetCode（数据库）- 树节点

题目链接：​​点击打开链接​​

题目大意：略。

解题思路：略。

AC 代码

-- 解决方案(1)
SELECT
    id, 'Root' AS Type
FROM
    tree
WHERE
    p_id IS NULL

UNION

SELECT
    id, 'Leaf' AS Type
FROM
    tree
WHERE
    id NOT IN (SELECT DISTINCT
            p_id
        FROM
            tree
        WHERE
            p_id IS NOT NULL)
        AND p_id IS NOT NULL

UNION

SELECT
    id, 'Inner' AS Type
FROM
    tree
WHERE
    id IN (SELECT DISTINCT
            p_id
        FROM
            tree
        WHERE
            p_id IS NOT NULL)
        AND p_id IS NOT NULL
ORDER BY id;

-- 解决方案(2)
SELECT
    id AS `Id`,
    CASE
        WHEN tree.id = (SELECT atree.id FROM tree atree WHERE atree.p_id IS NULL)
          THEN 'Root'
        WHEN tree.id IN (SELECT atree.p_id FROM tree atree)
          THEN 'Inner'
        ELSE 'Leaf'
    END AS Type
FROM
    tree
ORDER BY `Id`;

-- 解决方案(3)
SELECT
    atree.id,
    IF(ISNULL(atree.p_id),
        'Root',
        IF(atree.id IN (SELECT p_id FROM tree), 'Inner','Leaf')) Type
FROM
    tree atree
ORDER BY atree.id;

-- 解决方案(4)
SELECT DISTINCT t1.id, IF(t1.p_id IS NULL, "Root", IF(t2.id IS NOT NULL, "Inner", "Leaf")) Type
FROM tree t1 LEFT JOIN tree t2 ON t1.id = t2.p_id;