SQL - detecting loops in parent child relations

Question

I have parent child data in excel which gets loaded into a 3rd party system running MS SQL server. The data represents a directed (hopefully) acyclic graph. 3rd party mean

Answer 1

You can use a recursive CTE to detect loops:

with prev as (
    select RowId, 1 AS GenerationsRemoved
    from YourTable
    union all
    select RowId, prev.GenerationsRemoved + 1
    from prev
    inner join YourTable on prev.RowId = ParentRowId
    and prev.GenerationsRemoved < 55
)
select * 
from prev
where GenerationsRemoved > 50

This does require you to specify a maximum recursion level: in this case the CTE runs to 55, and it selects as erroneous rows with more than 50 children.