Cannot detect cyclic data in an SQLite database - sqlite

Question

Using SQLite, I have a suspected case of recursive data that has cycle - in other words, a child node is also its own grandparent. The symptom, of course, is an infinite loop
I know how Oracle and Postgresql handle cyclic data; but have not found any method to do so with SQLite. Below is an example of data with a cycle. If you remove the last "row" in the dataset table, it will work. As written, it goes into an infinite loop.
with DataSet as
(
select 'A' as node, null as parent union all
select 'B' as node, 'A' as parent union all
select 'C' as node, 'B' as parent union all
select 'D' as node, 'C' as parent union all
select 'A' as node, 'D' as parent
),
Hierarchy( node, parent, level, path )
as
(
select DataSet.node,
DataSet.parent,
1 as level,
'/' || DataSet.node as path
from DataSet
where DataSet.parent is null
union all
select DataSet.node,
DataSet.parent,
Hierarchy.level + 1 as level,
Hierarchy.path || '/' || DataSet.node as path
from Hierarchy
join DataSet
on DataSet.parent = Hierarchy.node
)
select *
from Hierarchy
order by path
;

Answer 1

Without the level, you could use UNION to ignore the duplicates.
Otherwise, there is no easy way to compare 'old' rows.
You could add a sufficiently large LIMIT to prevent the loop from becoming infinite, but then it is not always possible to remove duplicates.

Answer 2

To close the loop (pun intended) on this, I finally figured out a method that works. Plus I even added a column to show the rows where a cyclic condition exists. This solution includes: level (depth) number, parent, child, path (root to node/leaf), and a zero or one for detection of a cycle. You can paste the SQL below into the sqlite3 prompt and it will display:
level parent node path cyclic_flag
------ ------- ------- ------------ ------------
0 A B /A/B 0
1 B C /A/B/C 0
2 C D /A/B/C/D 0
3 D A /A/B/C/D/A 1
sqlite>
Finally, here is the SQL that shows the method. The only tricky part is the use of counting substrings in the path (the next node being the substring counted).
with dataset as
(
select 'A' as parent, 'B' as node union all
select 'B' as parent, 'C' as node union all
select 'C' as parent, 'D' as node
union all select 'D' as parent, 'A' as node
),
hierarchy( level, parent, node, path, cyclic_flag )
as
(
select 0 as level,
dataset.parent,
dataset.node,
'/' || dataset.parent || '/' || dataset.node as path,
0 as cyclic_flag
from dataset
where dataset.parent = 'A'
union all
select
hierarchy.level + 1 as level,
dataset.parent,
dataset.node,
hierarchy.path || '/' || dataset.node as path,
case
when
(length(path||dataset.node) - length(replace(path|| dataset.node,dataset.node,'')))/length(dataset.node) = 1
then 0
else 1
end as cyclic_flag
from hierarchy
inner join dataset
on dataset.parent = hierarchy.node
where (length(path) - length(replace(path,hierarchy.node,'')))/length(hierarchy.node) < 2
)
select *
from hierarchy
order by path
;