Question

所以我正在尝试编写一个递归查询来获取项目及其父项。问题是我的结构可以有很多父母，查询没有得到最后一行。结构是：

Entity

Id |
1  |
2  |
3  |


EntityMembership

ChildId | ParentId
3       | 2
2       | 1

到目前为止，查询看起来像：

with results(BaseId, Id, ParentId) as
(
 select e.id as BaseId, e.id, em.parentid
 from entity e
 left join EntityMembership em on e.Id = em.ChildID
 union all
 select r.BaseId, e.id, em.parentid
 from entity e
 inner join EntityMembership em on e.Id = em.ChildID
 inner join results r on r.parentid = e.id
 where r.Id<> r.ParentId
)
select results.Id
from results
where BaseId = 3

出现如下：

3 |
2 |

显然内部联接阻止“1”出现在结果中，因为它没有父级，但我不知道如何重写它以包含它。

Answer 1

问题是你在CTE的第二部分对EntityMembership进行了内部联接，所以你在结果中没有得到“1”。它应该是外连接（如第一部分中所示），但不允许在递归CTE中具有外连接。我的解决方案是提前在另一个CTE中加入Entity和EntityMembership，并在递归中使用它。

with joined(ChildId, ParentId) as
(select e.id, em.parentid
 from entity e 
 left outer join entitymembership em on em.childid = e.id
 ), 
 results(BaseId, Id, ParentId) as
(
 select j.ChildId as BaseId, j.ChildId, j.parentid
 from joined j 
 union all
 select r.BaseId, j.ChildId, j.parentid
 from joined j
 inner join results r on r.parentid = j.ChildId
 where r.Id <> r.ParentId
)

select Id
from results
where BaseId = 3

这是一种稍微不同的编写方式，过滤CTE中的子Id，而不是之后：

with joined(ChildId, ParentId) as
(select e.id, em.parentid
 from entity e 
 left outer join entitymembership em on em.childid = e.id
 )
,results(ChildId, ParentId) as
(
 select j.childid, j.parentid
 from joined j
 where j.childid = 3
 union all
 select j.childid, j.parentid
 from joined j
 join results r on r.parentid = j.childid 
)


select ChildId
from results

使用内部联接进行递归选择

1 个答案: