There is a table called User (userID PK, userName, supUserID FK). SupUserID refers to userID and contains the userID...
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value of the user's superior. The highest user has no user superior to itself (NULL instead of a value) and can have under itself one or more users or no users at all. Number of levels in such a tree is not defined.
Is there a way to count all users under the highest user by using SQL only (not Transact-SQL) or something else? Hope it's a solvable question in general.
In general, this requires recursion. Yours is the classic hierarchical structure, implemented using the adjacency model. As I mentioned in a previous answer, Recursion in SQL, only Oracle, to my knowledge, provides a way to do recursion.
Of course, if you can store your data using the nested set model, then the solution is straight-forward, I'm told (I don't actually use the nested set model myself). See Converting an adjacency list model to a nested set model.
If you must stick with the adjacency model, you can "work around" the need for recursion, provided you are able to set a realistic limit to the depth of the subtree. Just use an appropriate number of self-joins. If the path from the CEO to the lowliest cube jockey is eight layers of management, then have nine self-joins. Most databases are pretty good at joining tables, handling even a dozen or two with ease.
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