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Comments on How to break infinite loop in CTE
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How to break infinite loop in CTE
I have a parent-child relation in my table, with possibly circular cases. Is it possible to break the infinite recursion in CTE checking values of all previous rows? I would need something like this:
with my_cte(childId, parentId, col1)
AS (
SELECT r.childId, r.parentId, r.col1
FROM My_Table r WHERE r.childId = 'x' AND (r.col1 = 1 or r.col1 = 2)
UNION ALL
SELECT rel.childId, rel.parentId, rel.col1
FROM My_Table rel INNER JOIN my_cte sd ON rel.childId = sd.parentId where (rel.col1 = 1 OR rel.col1 = 2) AND ...
-- check that rel.childId, rel.parentId not in previous rows
)
SELECT DISTINCT * from shared_documents
How can I achieve this? Or how to approach this?
Example table with records would be:
| childId | parentId | col1 |
|---|---|---|
| 1 | 3 | 1 |
| 1 | 4 | 1 |
| 3 | 5 | 1 |
| 5 | 1 | 1 -- circular case |
| 6 | 2 | 1 |
| 5 | 7 | 1 |
If I ask for hierarchy related to 1, I should get
| childId | parentId | col1 |
|---|---|---|
| 1 | 3 | 1 |
| 1 | 4 | 1 |
| 3 | 5 | 1 |
| 5 | 1 | 1 -- circular case |
| 5 | 7 | 1 |
- 1 has two parents: 3 and 4.
- 3 has 1 parent: 5
- 5 has 2 parents: 1 and 7. But 1 brings the infinite loop, so the query should ignore this. And continue to the next row.
Post
Estela's answer provides great insight about how to do it also in SQL Server. Unfortunately, there does not seem to be a build-in array functionality, so one way is to rely on strings as shown here.
Basically, instead of accumulating values in an array, a string does this (way less efficiently for many records, I would say).
In this case, a query that seems to provide the results you are looking for is the following (some security check does not allow to post the query, so I am using SQL fiddle for sharing it):
http://sqlfiddle.com/#!18/a50101/1
Note: considering how complex things might get (edge cases might add up to the logic), this type of query might be better be done in a language/framework like C++, Java or .NET, unless there is a very strong reason to remain in SQL. These allow for much more flexibility, considering the iterative nature of the algorithm (not so suitable for set based operations) and also provide way better-debugging capabilities.
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