Flatten tree structure represented in SQL [duplicate] - sql

Question

This question already has an answer here:
SQL Server recursive self join
(1 answer)
Closed 3 years ago.
I'm using an engineering calculation package and trying to extract some information from it in a built in reporting tool that allows SQL query
An abbreviated example SQL tables are as follows:
Id | Description | Ref
---|---------------------
1 | system 1 |
3 | block 4 | 6
3 | block 4 | 1
5 | formula1 | 3
6 | f |
7 | something | 1
9 | cheese | 5
The "Ref" column identifies rows that are subrecords of other items.
What I want to do is run a query that will produce a list that will show all items that appear on a each page. As you can see from the table above "ID" is not the unique key; each item can appear in multiple locations within the table. In the example above:
ID 5 is a subitem of ID3
ID 3 is a subitem of ID 1 AND ID 6
ID 1 and ID 6 aren't subitems of anything
So effectively it is representing a tree structure:
ID 1
+-------- ID 7
|---- ID 3
+---- ID 5
+---- ID 9
ID 6
+---- ID 3
+---- ID 5
+---- ID 9
What I'm hoping to is work out which items appear under each top level item (so the end result should be a table where in the "Ref" column only top level items appear):
Id | Description | Ref
---|---------------------
1 | system 1 |
3 | block 4 | 6
3 | block 4 | 1
5 | formula1 | 1
5 | formula1 | 6
6 | f |
9 | cheese | 1
9 | cheese | 6
7 | something | 1
The tree structure can be a total of 5 levels deep
I've been trying to use left joins to build up a list of page references, but I think I'm also going to need to union results tables (because obviously rows like ID=9, ID=5, and ID = 6 have to be duplicated in the final results set). It starts to get a bit messy!
WITH A
AS (SELECT *
FROM [RbdBlocks]),
B
AS (SELECT [x].[Id],
[x].[Description],
[x].[Page] AS Page1,
[y].[Page] AS Page2,
FROM A AS x
LEFT OUTER JOIN
A AS y
ON y.Id = x.Page)
SELECT *
FROM B
The above gives me some of the nested references, but I'm not sure if there's a better way to get this data together, and to manage the recursion rather than just duplicating the set of queries 4 times?

Answer 1

Have a look at Recursive Common Table Expressions (CTEs). They should be able to accomplish exactly what you need.
Have a look at Example D on the SQL Docs page.
Basically what you'd do in your case is:
In the "anchor member" of the CTE, select all top-level items
In the "recursive member" of the CTE, join all of the nested children to the top-level item
Recursive CTEs are not really trivial to understand, so be sure to read the docs carefully.