Come posso scegliere la corrispondenza più vicina in SQL Server 2005?

per SQL Server 2005 e versioni successive, prova:

;WITH SalesScore AS (
SELECT
    s.PK_ID as S_PK
        ,c.PK_ID AS c_PK
        ,CASE 
             WHEN c.PK_ID IS NULL THEN 0
             ELSE CASE WHEN s.ServiceId=c.ServiceId THEN 1 ELSE 0 END
                  +CASE WHEN (s.Address=c.Address AND s.Zip=c.Zip) THEN 1 ELSE 0 END
                  +CASE WHEN s.EmailAddress=c.EmailAddress THEN 1 ELSE 0 END
                  +CASE WHEN s.HomePhone=c.HomePhone THEN 1 ELSE 0 END
         END AS Score
    FROM Sales s
        LEFT OUTER JOIN Customers c ON s.ServiceId=c.ServiceId
                                       OR (s.Address=c.Address AND s.Zip=c.Zip)
                                       OR s.EmailAddress=c.EmailAddress
                                       OR s.HomePhone=c.HomePhone 
)
SELECT 
    s.*,c.*
    FROM (SELECT
              S_PK,MAX(Score) AS Score
              FROM SalesScore 
              GROUP BY S_PK
         ) dt
        INNER JOIN Sales          s ON dt.s_PK=s.PK_ID 
        INNER JOIN SalesScore    ss ON dt.s_PK=s.PK_ID AND dt.Score=ss.Score
        LEFT OUTER JOIN Customers c ON ss.c_PK=c.PK_ID

MODIFICA Odio scrivere così tanto codice reale quando non è stato dato alcuno shema, perché non posso effettivamente eseguirlo ed essere sicuro che funzioni. Tuttavia, per rispondere alla domanda su come gestire i pareggi utilizzando la data dell'ultima transazione, ecco una versione più recente del codice precedente:

;WITH SalesScore AS (
SELECT
    s.PK_ID as S_PK
        ,c.PK_ID AS c_PK
        ,CASE 
             WHEN c.PK_ID IS NULL THEN 0
             ELSE CASE WHEN s.ServiceId=c.ServiceId THEN 1 ELSE 0 END
                  +CASE WHEN (s.Address=c.Address AND s.Zip=c.Zip) THEN 1 ELSE 0 END
                  +CASE WHEN s.EmailAddress=c.EmailAddress THEN 1 ELSE 0 END
                  +CASE WHEN s.HomePhone=c.HomePhone THEN 1 ELSE 0 END
         END AS Score
    FROM Sales s
        LEFT OUTER JOIN Customers c ON s.ServiceId=c.ServiceId
                                       OR (s.Address=c.Address AND s.Zip=c.Zip)
                                       OR s.EmailAddress=c.EmailAddress
                                       OR s.HomePhone=c.HomePhone 
)
SELECT
    *
    FROM (SELECT 
              s.*,c.*,row_number() over(partition by s.PK_ID order by s.PK_ID ASC,c.LastTransaction DESC) AS RankValue
              FROM (SELECT
                        S_PK,MAX(Score) AS Score
                        FROM SalesScore 
                        GROUP BY S_PK
                   ) dt
                  INNER JOIN Sales          s ON dt.s_PK=s.PK_ID 
                  INNER JOIN SalesScore    ss ON dt.s_PK=s.PK_ID AND dt.Score=ss.Score
                  LEFT OUTER JOIN Customers c ON ss.c_PK=c.PK_ID
         ) dt2
    WHERE dt2.RankValue=1