Question

我有一个如下表所示的表，它是树的表示。

id ---  parentid
 1        NULL
 2        1
 3        1
 4        2

这里1是根节点，4,3是叶节点，2是中间节点。如何编写打印

的SQL查询

id ---- type
1        root
2        intermiditae
3        leaf
4        leaf

Answer 1

这是一种方法：

SELECT mytable.id, IF(mytable.parent_id IS NULL, 'root',
                      IF(COUNT(children.id) > 0, 'intermediate', 'leaf'))
FROM mytable
LEFT JOIN mytable AS children ON children.parent_id = mytable.id
GROUP BY mytable.id;

＆＃34; root＆＃34;是parent_id为NULL的行。然后，计算孩子的数量足以确定该条目是否是叶子。

Try it online

您还可以使用3个查询和UNION获得相同的结果。

(SELECT mytable.id, 'root' AS type
 FROM mytable
 WHERE mytable.parent_id IS NULL)
UNION
(SELECT mytable.id, 'intermediate' AS type
 FROM mytable
 JOIN mytable AS children ON children.parent_id = mytable.id
 WHERE mytable.parent_id IS NOT NULL
 GROUP BY mytable.id)
UNION
(SELECT mytable.id, 'leaf' AS type
 FROM mytable
 LEFT JOIN mytable AS children ON children.parent_id = mytable.id
 WHERE children.id IS NULL);

Try it online

Answer 2

使用左连接来确定子项的存在：

select id,
  case when parent_id is null then 'root'
    when child is null then 'leaf'
    else 'intermediate' end as type
from mytable
left join (select parent_id parent, max(id) child
    from mytable group by 1) x on parent = id

MySQL案例陈述问题

2 个答案: