CTE 获取根名

Question

我得到了一个非常简单的 table，但我正在为 CTE 而苦苦挣扎：

ID | Parent | Name
---+--------+---------
 1 | NULL   | Root
 2 |   1    | Child1
 3 |   2    | Child2

我希望收到这样的结果：

Element | Root
--------+------
Root    | Root
Child1  | Root
Child2  | Root

我的 CTE 是这样的...

WITH cte AS
(
    SELECT a.id, a.parent, a.name, 1 as lvl
    FROM table1 a

    UNION ALL

    -- Perform the recursive join
    SELECT a.id, a.parent, a.name, Lvl+1 AS Lvl
    FROM table1 a
    INNER JOIN cte pa ON cte.parent = a.id
)
SELECT * 
FROM cte

现在我会汇总 (max) 和 (self)join，但看起来有点 bad/wrong。

Answer 1

这有点复杂，因为你正在遍历 ID，但最后你只需要名称。以下通过查找名称在递归 CTE 找到根 ID 后处理此问题：

with cte as (
      select id, parent, name, 1 as lev
      from t
      union all
      select cte.id, t.parent, cte.name, lev + 1
      from cte join
           t
           on t.id = cte.parent
      where t.parent is not null
     )
select top (1) with ties cte.name, coalesce(tparent.name, cte.name) as root
from cte left join
     t tparent
     on tparent.id = cte.parent
order by row_number() over (partition by cte.id order by cte.lev desc)

Here 是一个 db<>fiddle.

Answer 2

尝试

  with cte as (
      select id, parent, name, name as root
      from t
      where parent is null     
      union all
      select t.id, t.parent, t.name, cte.root
      from cte 
      join t on t.parent = cte.id
  )
  select name, root
  from cte

CTE 获取根名

CTE get rootname

sql

sql-server

common-table-expression