Can I select full hierarchy of parents when id and parent id are in the same table?

Question

I have a table which has a column for Id and parentId. ParentId contains the Id of another row in the table. If the ParentId is null then it is the top of the hierarchy.

Answer 1

Don't know LINQ, but as other answerers have written, many relational databases support Common Table Expressions (CTE) - but not all (Oracle comes to mind). And if supported, CTE is a good approach to retrieving the "ancestry".

That noted, there are some other approaches to consider in particular a bridge table or nested set. See my question for some explanation of these options and other ways of representing hierarchical data. Briefly, a bridge table most likely updated using CTE from a trigger will easily give you all ancestors or descendants - just not how close. A nested set model will give you this information and how close at the expense of more expensive inserts and updates comparatively.