sql - 将 Oracle CONNECT BY 迁移到 postgresql 10

Question

我有这个 SQL:

SELECT count(*) as ct
          FROM classifications cls
WHERE
   cls.classification_id = :classification_id
START WITH cls.classification_id = :root_classification_id
CONNECT BY NOCYCLE PRIOR cls.classification_id = cls.parent_id

并且需要将它迁移到 postgresql 10。

我已经安装了扩展 tablefunc 并尝试使用 connectedby。这是我的尝试:

SELECT count(*) as ct
          FROM classifications cls
WHERE
   cls.classification_id = :classification_id

union
                  SELECT count(classification_id) FROM connectby('classifications','classification_id','parent_id',:root_classification_id,5)
                  as t(classification_id varchar, parent_id varchar,level int) 

问题是并集的方式是错误的，因为那样你会得到 2 个计数结果。

Answer 1

无需使用 tablefunc 扩展。这可以使用 recursive CTE 轻松完成。

with recursive tree as (

  select cls.classification_id, cls.parent_id 
  from classifications cls
  where cls.classification_id = :root_classification_id

  union all

  select child.classification_id, child.parent_id
  from classifications child
     join tree parent on child.parent_id = parent.classification_id
)
select count(*)
from tree;

CTE 中的第一个查询匹配来自 Oracle 的 start with 的 start with 部分。第二个查询中返回 CTE 的 JOIN 匹配 Oracle 查询中的 connect by 部分。