Question

I’m not sure if such is possible in SQL Server’s database?

Say, I have a table:

id INT
nm NVARCHAR(256)
cid INT --references [id]

and hypothetical data:

id   nm       cid
1    Name 1   0
2    Name 2   0
3    Name 3   1
4    Name 4   3
5    Name 5   2
6    Name 6   4
7    Name 7   2

And the logic for selection should be as follows:

1. Say, we have the original id, let’s call it N.

2. Then we have a lookup id, let’s call it X. Then we do:

   SELECT [cid] FROM [TableName] WHERE [id]=X
   

and check if the result is equal to N. If yes, then we return [nm] for that record. If the result is 0, then we return Null. If result is something else we do the same selection, except X is now the result value.

I can obviously do this with C# but I’m curious if such is possible to wrap up in a pure SQL statement?

PS. Just to illustrate this with my table above, if N is 1 and X is 6, then we get:

SELECT [cid] FROM [TableName] WHERE [id]=6    --results is 4 (not N or 0, then continue)
SELECT [cid] FROM [TableName] WHERE [id]=4    --results is 3 (not N or 0, then continue)
SELECT [cid] FROM [TableName] WHERE [id]=3    --results is 1 (is N, then return "Name 1")

or if N is 1 and X is 7, we get:

SELECT [cid] FROM [TableName] WHERE [id]=7    --results is 2 (not N or 0, then continue)
SELECT [cid] FROM [TableName] WHERE [id]=2    --results is 0 (is 0, then return Null)

EDIT:
I need this to run under SQL Server 2008.

Answer 1

Recursive CTEs can do this. It would look like this:

WITH q AS (
    SELECT t.id, t.name, t.cid
    FROM t
    WHERE t.id = @x
    UNION ALL
    SELECT t.id, t.name, t.cid
    FROM q
    INNER JOIN t
    ON t.id = q.cid
)
SELECT name
FROM q
WHERE id = @n

The idea is that a CTE can refer to itself in the second part of a UNION ALL clause, and it will evaluate the second part until it generates no more results, or an internal limit is reached.