SQL Recursive CTE: Finding objects linked by property - sql-server

Question

I'm just trying to understand CTE and recursion to solve an issue that I would previously have used a cursor for.
create table ##ACC (
AccNo int,
Property char
)
Insert into ##ACC
VALUES (1,'A'),(1,'B'),(2,'A'),(2,'C'),(3,'C'),(4,'D')
What I'm trying to achieve is to get a list of all AccNo's, and all AccNo's they're related to via Property. So my expected results are
PrimaryAccNo | LinkedAccNo
1 | 1
1 | 2
1 | 3
2 | 1
2 | 2
2 | 3
3 | 1
3 | 2
3 | 3
4 | 4
I've attempted the following code and variations but I either get 4 results (PrimaryAccNo=LinkedAccNo) only or I hit 100 recursions.
WITH Groups(PrimaryAccNo, LinkedAccNo)
AS
(
Select distinct AccNo, AccNo from ##ACC
UNION ALL
Select g.PrimaryAccNo, p.AccNo from
##ACC p inner join Groups g on p.AccNo=g.LinkedAccNo
inner join ##ACC pp on p.Property=pp.Property
where p.AccNo<> pp.AccNo
)
Select PrimaryAccNo,LinkedAccNo
from Groups
What am I doing wrong?

Answer 1

You're running into an infinite loop caused by cycles within your data, e.g.: 1 > 2 > 3 > 2 > ... . The solution is to keep track of the rows that have already been "consumed". Due to limitations in CTEs, this has to be done by including the history within each CTE row, e.g. by assembling the path followed to arrive at each row. You can uncomment the , Path on the final select to see what is going on.
-- Sample data.
declare #ACC as Table ( AccNo Int, Property Char );
insert into #ACC values
( 1, 'A' ), ( 1, 'B' ), ( 2, 'A' ), ( 2, 'C' ), ( 3, 'C' ), ( 4, 'D' );
select * from #ACC;
-- Recursive CTE.
with Groups as (
select distinct AccNo, AccNo as LinkedAccNo,
Cast( '|' + Cast( AccNo as VarChar(10) ) + '|' as VarChar(1024) ) as Path
from #ACC
union all
select G.AccNo, A.AccNo, Cast( Path + Cast( A.AccNo as VarChar(10) ) + '|' as VarChar(1024) )
from Groups as G inner join -- Take the latest round of new rows ...
#ACC as AP on AP.AccNo = G.LinkedAccNo inner join -- ... and get the Property for each ...
#ACC as A on A.Property = AP.Property -- ... to find new linked rows.
where G.Path not like '%|' + Cast( A.AccNo as VarChar(10) ) + '|%' )
select AccNo, LinkedAccNo -- , Path
from Groups
order by AccNo, LinkedAccNo;

Answer 2

Another approach similar to yours but differs in the following:
The property value is included in the recursive CTE so that it can be used later
The < is used to prevent duplicates and the resulting infinite recursion
Another CTE is added AccGroups to provide the mirror of the relations
A demo fiddle has been included below:
CREATE TABLE ##ACC (
AccNo int,
Property char
);
INSERT INTO ##ACC
VALUES (1,'A'),(1,'B'),(2,'A'),(2,'C'),(3,'C'),(4,'D');
WITH Groups(PrimaryAccNo, LinkedAccNo, Property) AS (
SELECT AccNo, AccNo, Property FROM ##ACC
UNION ALL
SELECT g.PrimaryAccNo, pp.AccNo, pp.Property
FROM Groups g
INNER JOIN ##ACC p ON g.Property=p.Property AND
g.LinkedAccNo < p.AccNo
INNER JOIN ##ACC pp ON p.AccNo = pp.AccNo
),
AccGroups AS (
SELECT DISTINCT * FROM (
SELECT PrimaryAccNo, LinkedAccNo FROM Groups
UNION ALL
SELECT LinkedAccNo, PrimaryAccNo FROM Groups
) t
)
SELECT * FROM AccGroups
ORDER BY PrimaryAccNo,LinkedAccNo
GO
PrimaryAccNo | LinkedAccNo
-----------: | ----------:
1 | 1
1 | 2
1 | 3
2 | 1
2 | 2
2 | 3
3 | 1
3 | 2
3 | 3
4 | 4
db<>fiddle here