Hier ist eine vollständige Lösung mit einem rekursiven CTE.
;WITH Nodes AS
(
SELECT DENSE_RANK() OVER (ORDER BY Part, PartRank) SetId
, [ID]
FROM
(
SELECT [ID], 1 Part, DENSE_RANK() OVER (ORDER BY [E-mail]) PartRank
FROM dbo.Customer
UNION ALL
SELECT [ID], 2, DENSE_RANK() OVER (ORDER BY Phone) PartRank
FROM dbo.Customer
) A
),
Links AS
(
SELECT DISTINCT A.Id, B.Id LinkedId
FROM Nodes A
JOIN Nodes B ON B.SetId = A.SetId AND B.Id < A.Id
),
Routes AS
(
SELECT DISTINCT Id, Id LinkedId
FROM dbo.Customer
UNION ALL
SELECT DISTINCT Id, LinkedId
FROM Links
UNION ALL
SELECT A.Id, B.LinkedId
FROM Links A
JOIN Routes B ON B.Id = A.LinkedId AND B.LinkedId < A.Id
),
TransitiveClosure AS
(
SELECT Id, Id LinkedId
FROM Links
UNION
SELECT LinkedId Id, LinkedId
FROM Links
UNION
SELECT Id, LinkedId
FROM Routes
),
UniqueCustomers AS
(
SELECT Id, MIN(LinkedId) UniqueCustomerId
FROM TransitiveClosure
GROUP BY Id
)
SELECT A.Id, A.[E-mail], A.Phone, B.UniqueCustomerId
FROM dbo.Customer A
JOIN UniqueCustomers B ON B.Id = A.Id
