问题
In Oracle, we can use the function CONNECT_BY_ISCYCLE to detect a cycle in Hierarchical Queries. I try to do the same in SQL Server. Is there a way to do this?
Thanks a lot!
回答1:
Concatenate the records IDs / build a bitmap based on ROW_NUMBERs of the records and verify each new record against the list/bitmap
create table t (id int,pid int)
insert into t values (1,3),(2,1),(3,2)
List
Identify Cycles
with cte (id,pid,list,is_cycle)
as
(
select id,pid,',' + cast (id as varchar(max)) + ',',0
from t
where id = 1
union all
select t.id,t.pid,cte.list + cast (t.id as varchar(10)) + ',' ,case when cte.list like '%,' + cast (t.id as varchar(10)) + ',%' then 1 else 0 end
from cte join t on t.pid = cte.id
where cte.is_cycle = 0
)
select *
from cte
where is_cycle = 1
id pid list is_cycle
-- --- ---- --------
1 3 ,1,2,3,1, 1
Traverse Thorough Graph with Cycles
with cte (id,pid,list)
as
(
select id,pid,',' + cast (id as varchar(max)) + ','
from t
where id = 1
union all
select t.id,t.pid,cte.list + cast (t.id as varchar(10)) + ','
from cte join t on t.pid = cte.id
where cte.list not like '%,' + cast (t.id as varchar(10)) + ',%'
)
select *
from cte
id pid list
1 3 ,1,
2 1 ,1,2,
3 2 ,1,2,3,
Bitmap
ID should be a sequence of numbers starting with 1.
If necessary generate it using ROW_NUMBER.
Identify Cycles
with cte (id,pid,bitmap,is_cycle)
as
(
select id,pid,cast (power(2,id-1) as varbinary(max)) ,0
from t
where id = 1
union all
select t.id,t.pid,cast (cte.bitmap|power(2,t.id-1) as varbinary(max)),case when cte.bitmap & power(2,t.id-1) > 0 then 1 else 0 end
from cte join t on t.pid = cte.id
where cte.is_cycle = 0
)
select *
from cte
where is_cycle = 1
id pid bitmap is_cycle
1 3 0x00000007 1
Traverse Thorough Graph with Cycles
with cte (id,pid,bitmap)
as
(
select id,pid,cast (power(2,id-1) as varbinary(max))
from t
where id = 1
union all
select t.id,t.pid,cast (cte.bitmap|power(2,t.id-1) as varbinary(max))
from cte join t on t.pid = cte.id
where cte.bitmap & power(2,t.id-1) = 0
)
select *
from cte
id pid bitmap
1 3 0x00000001
2 1 0x00000003
3 2 0x00000007
回答2:
If you look at a specific path across a hierarchy, you can say that it is a singled/doubled linked list (normally singled).
One easy way of making sure that you don't have a close loop, it to traverse the chain through two paths, each with its own index, one that advances by one position while the other by two.
If there is no closed loop, one of the indices will fall at some point (e.g. will reach the root node of the hierarchy which does not have any parent). If there is a loop, you will reach a point where the two indices point to the same node within the chain.
This is a rather old method but works beautifully.
来源：https://stackoverflow.com/questions/40574229/is-there-a-way-to-detect-a-cycle-in-hierarchical-queries-in-sql-server