Hierarchy Without CTE - Get Direct Children - sql

Question

I have a table for assets:
id|name|parentId
The view I'm trying to build is for an asset is:
{
'Id': ......,
'Name': ....,
'ChildrenIds': []
}
I need a query that selects TOP 50 assets and its direct children (so results could be more than 50 results).
I have a CTE that works, but its slow (5 seconds, parentId & id is indexed):
WITH MyCte as
(
SELECT TOP 50 a.Id, a.Name, a.ParentAssetId
FROM assets a
UNION ALL
SELECT a2.AssetId, a2.ParentAssetId
FROM assets a2
INNER JOIN MyCte cte ON cte.Id = a2.ParentAssetId
)
SELECT * From MyCte;
This join query does half of what I want.
SELECT TOP 50 a.Id, a.Name, a.ParentAssetId
FROM assets a
LEFT JOIN assets a2 ON a2.ParentAssetId = a.Id
Problem with JOIN, it gives me 50 results, and that's it. I need the descendant info to build a view. I could do 2 queries, but I'd rather not do that.
Any suggestions?
Maybe there is a better way for me to build this view? Without the 50 + N requirement? You can use a GROUP BY with STRING_AGG, but I worry about the size limitation.
SAMPLE DATA:
1,Site1,NULL
2,Site2,1
3,Site3,1
4,Site4,2
5,Site5,NULL
TOP 3 ORDER BY id DESC results will return
1,Site1,NULL
2,Site2,1
3,Site3,1
4,Site4,2
BUT I guess ideally something like this:
1,Site1,NULL|2,Site2,1|3,Site3,1
2,Site2,1|4,Site4,2
3,Site3,1

Answer 1

You can use a temp table to achieve what you need.
SELECT TOP (50) a.Id, a.Name, a.ParentAssetId
INTO #Assets
FROM assets a;
INSERT INTO #Assets
SELECT a2.Id, a2.Name, a2.ParentAssetId
FROM #Assets a
JOIN assets a2 ON a2.ParentAssetId = a.Id;
SELECT *
FROM #Assets;
Note that this is not deterministic because there's no ORDER BY when using TOP.

Answer 2

You could use this CTE and make a view from it:
WITH MyCte as
(
SELECT TOP 50 a.Id, a.Name, a.ParentAssetId
FROM assets a
)
SELECT cte.*, a1.Id as ChildId, a1.Name as ChildName
FROM MyCte cte
INNER JOIN assets a1
ON a1.ParentAssetId=cte.Id
Admittedly this will give you a different kind of result set than the UNION CTE in your question, but I'm assuming that you can make a simple adjustment to your consumer application to handle it. It might even be easier/more performant for the app this way, since the relationships are present in the row, and don't have to be extrapolated.
That said, if you are working with a recent-enough Version of SQL Server, you might look into the built-in JSON functions, since it looks like that is the output you are ultimately trying to generate.

Answer 3

According to what you provide, and if I understand, I think you're looking for
WITH CTE AS
(
SELECT TOP 3 *
FROM T
ORDER BY ID DESC
)
SELECT *
FROM CTE
UNION
SELECT *
FROM T
WHERE ID IN (SELECT ParentId FROM CTE);
Returns:
+----+-------+----------+
| ID | Name | ParentId |
+----+-------+----------+
| 1 | Site1 | |
| 2 | Site2 | 1 |
| 3 | Site3 | 1 |
| 4 | Site4 | 2 |
| 5 | Site5 | |
+----+-------+----------+
Here is a db<>fiddle
UPDATE:
Since you're looking for a way to pass an INT value present the rows number used in TOP, you can create an inline table-valued function as
CREATE FUNCTION dbo.MyFunction (#Rows INT = 1)
RETURNS TABLE
AS
RETURN
(
WITH CTE AS
(
SELECT TOP (#Rows) *
FROM T
ORDER BY ID DESC
)
SELECT *
FROM CTE
UNION
SELECT *
FROM T
WHERE ID IN (SELECT ParentId FROM CTE)
);
and just call it as
SELECT *
FROM dbo.MyFunction(2)
Demo