Recursive query with parent-child relation - sql

Question

I am trying to make a recursive query in SQL Server, that display data hierarchically. Here is the structure of the table
[id] [int] IDENTITY(1,1) NOT NULL,
[name] [varchar(100)] NOT NULL,
[Parent_Id] [int] NULL,
Each product has a parent. The column Parent_Id content the id of the parent. The parent_id is null for root products.
I want to make a sql query that display products hierarchically. The following image is an example of how the products could be organized.
Products can have products childs.
For the picture above, the query result should be like the following :
id name parent_id
1 P1 NULL
2 P2 NULL
3 P2-1 2
4 P2-2 2
5 P2-3 2
6 P2-3-1 5
7 P2-3-2 5
8 P3 NULL
9 P3-1 8
Here is the request I wrote to achieve it :
with tree as (select * from products
union all
select * from tree where parent_id = tree.id
)
select * from tree;
But I get a result similar to the following:
1 P1 NULL
2 P2 NULL
8 P3 NULL
3 P2-1 2
4 P2-2 2
5 P2-3 2
9 P3-1 8
6 P2-3-1 5
7 P2-3-2 5
What I want is to group each products sibling so that each product is displayed under its direct parent.

Answer 1

Just another option using the data type hierarchyid
There are some additional features and functions associated with hierarchyid
Example
-- Optional See 1st WHERE
Declare #Top int = null --<< Sets top of Hier Try 2
;with cteP as (
Select ID
,parent_id
,Name
,HierID = convert(hierarchyid,concat('/',ID,'/'))
From YourTable
Where IsNull(#Top,-1) = case when #Top is null then isnull(parent_id ,-1) else ID end
--Where parent_id is null -- Use this where if you always want the full hierarchy
Union All
Select ID = r.ID
,parent_id = r.parent_id
,Name = r.Name
,HierID = convert(hierarchyid,concat(p.HierID.ToString(),r.ID,'/'))
From YourTable r
Join cteP p on r.parent_id = p.ID)
Select Lvl = HierID.GetLevel()
,ID
,parent_id
,Name
From cteP A
Order By A.HierID
Results
Lvl ID parent_id Name
1 1 NULL P1
1 2 NULL P2
2 3 2 P2-1
2 4 2 P2-2
2 5 2 P2-3
3 6 5 P2-3-1
3 7 5 P2-3-2
1 8 NULL P3
2 9 8 P3-1
Just for fun, If I set #Top to 2, the results would be
Lvl ID parent_id Name
1 2 NULL P2
2 3 2 P2-1
2 4 2 P2-2
2 5 2 P2-3
3 6 5 P2-3-1
3 7 5 P2-3-2

Answer 2

Construct the path in the recursive query. The following does this as a string with fixed length ids:
with tree as (
select p.id, p.name, p.parentid,
format(p.parentid, '0000') as path
from products p
where p.parentid is null
union all
select p.id, p.name, p.parentid,
concat(cte.path, '->', format(p.id, '0000')
from tree join
products p
where p.parent_id = t.id
)
select *
from tree;

Answer 3

If I understand this correct and you have the result you want but just unordered, you should be able to just order the result by name.
with tree as (select * from products
union all
select * from tree where parent_id = tree.id
)
select * from tree order by name asc;