一对多连接按优先级获取一行

Question

我有一个联系人表：

ID  NAME
--- ----
1   KK
2   JKI
3   HU

我有一张电话桌：

ID  ContactID  Phone  Type
--- ---------  -----  --------
1   1          569    Business
2   1          896    Mobile
3   1          258    Fax
4   2          369    Mobile
5   3          124    Fax
6   2          496    Fax

我想获取至少有一个电话号码的所有联系人。要显示的电话号码应为“商务”，如果没有可用的商务类型，则为“移动”，如果没有可用的“移动”类型，则为“传真”，否则为 null

结果示例：

ID  NAME   PHONE
--- ------ ------
1   KK     569   -- Business present
2   JKI    369   -- Business not present but mobile present
3   HU     124   -- only fax present

Answer 1

;WITH [prior](i,t) AS 
(
    SELECT 1, 'Business'
    UNION ALL SELECT 2, 'Mobile'
    UNION ALL SELECT 3, 'Fax'
),
x AS 
(
  SELECT c.ID, c.Name, p.Phone,
    rn = ROW_NUMBER() OVER (PARTITION BY c.ID ORDER BY r.i)
  FROM dbo.Contacts AS c
  LEFT OUTER JOIN dbo.Phone AS p
  ON p.ContactID = c.ID
  LEFT OUTER JOIN [prior] AS r
  ON r.t = p.[Type]
)
SELECT ID, Name, Phone FROM x
WHERE rn = 1;

如果您想消除没有电话的联系人，只需将两个实例更改LEFT OUTER为INNER。