如何在mySQL中的子查询中指定父查询字段？

Question

有没有办法在mySQL的子查询中指定父查询字段？

例如:
我用PHP编写了一个基本的公告板类型程序.

在数据库中,每个帖子包含:id(PK)和parent_id(父帖子的id).如果帖子本身是父级,则其parent_id设置为0.

我正在尝试编写一个mySQL查询,它将查找每个父帖子以及父母拥有的子节点数.

$query = "SELECT id, (
      SELECT COUNT(1) 
      FROM post_table 
      WHERE parent_id = id
) as num_children
FROM post_table
WHERE parent_id = 0";

棘手的部分是第一个id不知道它应该引用子查询之外的第二个id.我知道我可以做SELECT id AS id_tmp然后在子查询中引用它,但是如果我还想返回id并保持"id"作为列名,那么我必须做一个返回的查询我有2列相同的数据(这对我来说似乎很乱)

$query = "SELECT id, id AS id_tmp, 
            (SELECT COUNT(1)
            FROM post_table
            WHERE parent_id = id_tmp) as num_children
         FROM post_table
         WHERE parent_id = 0";

凌乱的方式很好,但我觉得有机会在这里学到一些东西,所以我想我会发布这个问题.

Answer 1

怎么样:

$query = "SELECT p1.id, 
                 (SELECT COUNT(1) 
                    FROM post_table p2 
                   WHERE p2.parent_id = p1.id) as num_children
            FROM post_table p1
           WHERE p1.parent_id = 0";

或者如果你在p1.id上添加别名,你可能会说:

$query = "SELECT p1.id as p1_id, 
                 (SELECT COUNT(1) 
                    FROM post_table p2 
                   WHERE p2.parent_id = p1.id) as num_children
            FROM post_table p1
           WHERE p1.parent_id = 0";

Answer 2

你可以试试这样的

SELECT  pt.id,
        CountTable.Cnt
FROM    post_table pt LEFT JOIN
        (
            SELECT  parent_id,
                    COUNT(1) Cnt
            FROM    post_table
            WHERE   parent_id <> 0
            GROUP BY parent_id
        ) CountTable ON pt.id = CountTable.parent_id
WHERE   pt.parent_id = 0

要回到您的示例，请在子选择中使用主表的别名

SELECT  pt.id,
        (SELECT COUNT(1) FROM post_table WHERE parent_id = pt.id) 
FROM    post_table pt
WHERE   pt.parent_id = 0