Retrieving a tree of references with SQL Server - sql

Question

I've got a database that manages files - some files contain/reference other files, and my goal is to design a query that can give me the whole "tree" for a given document.
For example the structure may look like this:
File 1
File 2
File 3
File 4
File 5
File 6
File 7
File 8
File 9
File 10
etc., where File 1 effectively contains all of the files following it
These are broken out in my database between two tables - lets call them the "Files" table and the "References" table
The "Files" table has information about the files themselves - FileID, Filename, etc.
The "References" table shows the relationship of the above structure using the FileIDs of the files. My issue is that, for example, File 6 is not referenced by File 1 - it is only referenced by File 5.
e.g.:
[ParentFileID] [ChildFileID]
1 2
1 3
1 4
1 5
5 6
5 7
5 8
8 9
8 10
Ideally I'd like to be able to check the position in the entire structure for any given FileID I pass in
Any ideas? I've been reading up on CTEs and it feels like some sort of recursive common table expression is what I'm after, though every example I can find is using one table and involves NULLs to track down the top level elements.

Answer 1

Yes, it can be done using a recursive CTE.
USE tempdb
GO
CREATE TABLE files
(
[file_id] int PRIMARY KEY,
[file_name] varchar(128) NOT NULL
);
INSERT INTO files VALUES
(1, 'File 1'),
(2, 'File 2'),
(3, 'File 3'),
(4, 'File 4'),
(5, 'File 5'),
(6, 'File 6'),
(7, 'File 7'),
(8, 'File 8'),
(9, 'File 9'),
(10, 'File 10');
CREATE TABLE [references]
(
parent_file_id int NOT NULL,
child_file_id int NOT NULL,
PRIMARY KEY (child_file_id)
);
INSERT INTO [references] VALUES
(1, 2),
(1, 3),
(1, 4),
(1, 5),
(5, 6),
(5, 7),
(5, 8),
(8, 9),
(8, 10);
GO
CREATE FUNCTION dbo.get_file_with_path(#file_id int)
RETURNS TABLE
AS
RETURN WITH h
AS
(
SELECT
f.file_id, f.file_id as child_file_id,
f.file_name, 0 as reverse_level,
CAST( '/' + f.file_name as varchar(8000)) as path
FROM
dbo.files f
WHERE
f.file_id = #file_id
UNION ALL
SELECT
h.file_id, r.parent_file_id as child_file_id,
h.file_name, h.reverse_level + 1 as reverse_level,
CAST('/' + f.file_name + h.path as varchar(8000)) as path
FROM
h
INNER JOIN [references] r
ON h.child_file_id = r.child_file_id
INNER JOIN dbo.files f
ON f.file_id = r.parent_file_id
)
SELECT TOP(1) h.file_id, h.file_name, h.path
FROM h
ORDER BY h.reverse_level DESC;
GO
SELECT *
FROM dbo.get_file_with_path(1)
UNION ALL
SELECT *
FROM dbo.get_file_with_path(3)
UNION ALL
SELECT *
FROM dbo.get_file_with_path(6)
UNION ALL
SELECT *
FROM dbo.get_file_with_path(10)
Output:
| file_id | file_name | path |
|---------|-----------|-------------------------------|
| 1 | File 1 | /File 1 |
| 3 | File 3 | /File 1/File 3 |
| 6 | File 6 | /File 1/File 5/File 6 |
| 10 | File 10 | /File 1/File 5/File 8/File 10 |
I assume you mean path when you say position
EDIT:
Answering the question in comments, you can also create a table valued function that returns the sub-tree below a given node:
CREATE FUNCTION dbo.get_file_subtree_excluding_self(#file_id int)
RETURNS TABLE
AS RETURN
WITH h AS
(
SELECT r.parent_file_id, r.child_file_id
FROM [references] r
WHERE r.parent_file_id = #file_id
UNION ALL
SELECT r.parent_file_id, r.child_file_id
FROM
h INNER JOIN [references] r
ON h.child_file_id = r.parent_file_id
)
SELECT h.child_file_id as [file_id]
FROM h
GO
SELECT * FROM dbo.get_file_subtree_excluding_self(5)
Output:
+---------+
| file_id |
+---------+
| 6 |
| 7 |
| 8 |
| 9 |
| 10 |
+---------+
The table references describes a graph. One node can have only one parent because of the primary key, but nothing prevents cycles. For example, consider the following data:
+-------+--------+
| child | parent |
+-------+--------+
| 1 | 2 |
| 2 | 3 |
| 3 | 1 |
+-------+--------+
As you can see, there are cycles on this graph.