Question

我目前有一个MySQL论坛帖子。这包括父帖和子帖。如果是父帖子，则其为topic。如果是儿童帖子，则其中包含parent_id。

E.g：

TABLE posts

+----+-----------+---------+--------+------+
| id | parent_id |  topic  | author | body |
+----+-----------+---------+--------+------+
|  1 |      NULL | "Hello" |  "Me"  | ...  |
|  2 |         1 |  NULL   |  "Me"  | ...  |
+----+-----------+---------+--------+------+

我想运行一个SQL查询，类似于以下内容：

SELECT id, 'self' AS parent_id, topic, author, body FROM posts WHERE parent_id IS NULL,
UNION
SELECT id, parent_id, (SELECT topic WHERE id=parent_id) as topic FROM posts WHERE topic IS NULL

期望的输出：

+----+-----------+---------+--------+------+
| id | parent_id |  topic  | author | body |
+----+-----------+---------+--------+------+
|  1 |    "self" | "Hello" |  "Me"  | ...  |
|  2 |         1 | "Hello" |  "Me"  | ...  |
+----+-----------+---------+--------+------+

基本上，我想返回父主题，而不必运行多个查询。有没有办法做到这一点？我应该只添加一个“parent_topic”字段吗？

谢谢！

Answer 1

以下SQL将执行您正在寻找的内容：

select p_child.id, if(p_child.parent_id is null, 'self', p_child.parent_id) as parent_id, if(p_parent.id is null, p_child.topic, p_parent.topic) as topic, p_child.author, p_child.body
from posts p_child
    left join posts p_parent on p_child.parent_id = p_parent.id;

Answer 2

尝试此查询，如果parent_id is not null，我在子查询中选择父项topic。我正在使用case来做到这一点。

SELECT id,
    CASE WHEN parent_id IS NULL THEN 'SELF'
         ELSE parent_id
    END as parent_id
    CASE WHEN parent_id IS NOT NULL THEN (SELECT t.topic FROM posts as t WHERE t.id = m.parenyt_id)
        ELSE topic
    END as Topic,
    author,
    body
FROM posts as m

MySQL - SELECT ...（子查询）AS fieldname

2 个答案: