SQL to retrieve all linked records - sql

Question

I have a product table (tProduct) and a product links table (tProductLink) to allow establishing links between products. Given a ProductID and ProductLinkID, I need to get all of the tProduct.ID records that are related.
In the example table (tProductLink) below, all of the ID's would be returned. Note that it's not possible to create a recursive link; that is given the first row in the table below there cannot be a row where ProductID is 31563 and ProductID is 28818.
So say I search for all products related to the link in row 4, ProductID 137902 and LinkProductID 410901. Give that link, it should return all six rows.
Here is an example of the data.
I have tried various techniques such as a recursive CTE and calling a table function using "cross apply" but I have got nowhere.
This is one of the last solutions I tried, which ended up not returning all products as noted in the comments.
declare #ProductID int, #ProductLinkID int
select #ProductID = 137902
select #ProductLinkID = 410901
;with p1 as
(
select ProductID, ProductLinkID
from tProductLink
where ProductID = #ProductID and ProductLinkID = #ProductLinkID
union all
select tProductLink.ProductID, tProductLink.ProductLinkID
from tProductLink
join p1 on p1.ProductLinkID = tProductLink.ProductID
where not (tProductLink.ProductID = #ProductID and tProductLink.ProductLinkID = #ProductLinkID)
)
select distinct ProductID from p1
union
select ProductLinkID from p1

Answer 1

You start with one ID. This can be in multiple rows ProductLinkId or ProductId in the second table. You look up the corresponding IDs thus found again in the second table.
This asks for a recursive query, where you always collect all corresponding IDs. Unfortunately SQL Server does not support DISTINCT in recursive queries, so the same IDs get looked up multiple times. SQL Server also doesn't prevent from cycles (but fails instead), so we must prevent them ourselves by remembering which IDs we already found. This would ideally be done with an array or set that we fill, but SQL Server doesn't support such, so we must build a string instead.
The complete query:
with cte(id, seen) as
(
select 28520 as id, cast('/28520/' as varchar(max)) as seen from t1
union all
select case when cte.id = t2.productid then t2.linkproductid
else t2.productid end as id,
cte.seen + cast(case when cte.id = t2.productid
then t2.linkproductid
else t2.productid end as varchar(max)) + '/'
from cte
join t2 on cte.id in (t2.productid, t2.linkproductid)
and charindex('/' + cast(case when cte.id = t2.productid
then t2.linkproductid
else t2.productid end as varchar(max))+ '/', cte.seen) = 0
)
select distinct id from cte
option (maxrecursion 1000);
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