如何将LEFT JOIN限制为SQL Server中的第一个结果？

时间：2010-07-30 21:21:04

标签： sql sql-server sql-server-2000 greatest-n-per-group

我有一点SQL几乎正在做我想做的事情。我正在使用三个表，一个是Users，UserPhoneNumbers和UserPhoneNumberTypes。我正在尝试获取用户导出的电话号码列表。

数据库本身很旧并且存在一些完整性问题。我的问题是，数据库中每个电话号码应该只有1种类型，但事实并非如此。当我运行这个时，我得到每个人的多行结果，如果它们包含例如两个“Home”数字。

如何修改SQL以获取列出的第一个电话号码并忽略其余的号码？我在SQL Server中，我知道TOP语句。但是如果我将'TOP 1'添加到LEFT JOIN select语句中，它只是给我数据库中的第一个条目，而不是每个用户的第一个条目。

这适用于SQL Server 2000。

谢谢，

SELECT  Users.UserID, 
  Users.FirstName, Users.LastName,
  HomePhone, WorkPhone, FaxNumber

FROM Users

LEFT JOIN
 (SELECT UserID, PhoneNumber AS HomePhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Home') AS tmpHomePhone
 ON tmpHomePhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, PhoneNumber AS WorkPhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Work') AS tmpWorkPhone
 ON tmpWorkPhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, PhoneNumber AS FaxNumber
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Fax') AS tmpFaxNumber
 ON tmpFaxNumber.UserID = Users.UserID

8 个答案:

答案 0 :(得分：7)

假设SQL Server 2005+，请使用ROW_NUMBER：

LEFT JOIN (SELECT UserID, 
                  PhoneNumber AS HomePhone,
                  ROW_NUMBER() OVER (PARTITION BY userid ORDER BY what?) AS rank
             FROM UserPhoneNumbers  upn
        LEFT JOIN UserPhoneNumberTypes upnt ON upnt.UserPhoneNumberTypeID = upn.UserPhoneNumberTypeID
                                           AND upnt.PhoneNumberType='Home') AS tmpHomePhone
                ON tmpHomePhone.UserID = Users.UserID
               AND tmpHomePhone.rank = 1

注意what?占位符以确定第一个数字。如果你根本不关心，请省略ORDER BY ......

答案 1 :(得分：7)

如果您只想在右表中为每行选择左表中的顶行，您应该考虑使用APPLY运算符而不是join，并将连接条件移到里面左连接：

SELECT  u.UserID, 
  u.FirstName, u.LastName,
  hn.PhoneNumber AS HomePhone
FROM Users u
OUTER APPLY (
 SELECT TOP(1) PhoneNumber 
 FROM UserPhoneNumbers upn
 LEFT JOIN UserPhoneNumberTypes upt 
   ON upn.UserPhoneNumberTypeID=upt.UserPhoneNumberTypeID
 WHERE upt.PhoneNumberType='Home'
 AND upn.UserID = u.UserID
 ORDER BY ...) as hn
...

答案 2 :(得分：6)

由于它的SQL Server 2000和排名函数已经用完，你可以使子查询SELECTs聚合：

SELECT UserID, MAX(PhoneNumber) AS HomePhone FROM [...] GROUP BY UserID

如果您不在乎返回用户的主页号码...

答案 3 :(得分：1)

我假设您在每个连接表上都有一些主键字段，因为UserID不是唯一的。我假设你的主键叫做ID。我们将采用ID最低的记录。这符合您的“第一”标准。

SELECT  Users.UserID, Users.FirstName, Users.LastName, hp.HomePhone,
        wp.WorkPhone, fn.FaxNumber
FROM Users
LEFT JOIN HomePhone hp ON hp.UserID = Users.UserID
LEFT JOIN HomePhone hp2 ON hp2.UserID = Users.UserID AND hp2.ID < hp.ID
LEFT JOIN WorkPhone wp ON wp.UserID = Users.UserID
LEFT JOIN WorkPhone wp2 ON wp2.UserID = Users.UserID AND wp2.ID < wp.ID
LEFT JOIN FaxNumber fn ON fn.UserID = Users.UserID
LEFT JOIN FaxNumber fn2 ON fn2.UserID = Users.UserID AND fn2.ID < fn.ID
WHERE hp2.ID IS NULL AND wp2.ID IS NULL AND fn2.ID IS NULL

在SQL Antipatterns一书中，有一整章关于这类问题，称为“模糊的Gruops”。

答案 4 :(得分：1)

Select Users.UserID,  Users.FirstName, Users.LastName
    , PhoneNumbers.HomePhone
    , PhoneNumbers.WorkPhone
    , PhoneNumbers.FaxNumber
From Users
    Left Join   (
                Select UPN.UserId
                    , Min ( Case When PN.PhoneNumberType = 'Home' Then UPN.PhoneNumber End ) As HomePhone
                    , Min ( Case When PN.PhoneNumberType = 'Work' Then UPN.PhoneNumber End ) As WorkPhone
                    , Min ( Case When PN.PhoneNumberType = 'Fax' Then UPN.PhoneNumber End ) As FaxPhone
                From UserPhoneNumbers As UPN
                        Join    (
                                Select Min(UPN1.UserPhoneNumberId) As MinUserPhoneNumberId
                                    , UPNT1.PhoneNumberType
                                From UserPhoneNumbers As UPN1
                                    Join UserPhoneNumberTypes As UPNT1
                                        On UPNT1.UserPhoneNumberTypeID = UPN1.UserPhoneNumberTypeID
                                Where UPNT1.PhoneNumberType In('Home', 'Work', 'Fax')
                                Group By UPN1.UserID, UPNT.PhoneNumberType
                                ) As PN
                            On PN.MinUserPhoneNumberId = UPN.UserPhoneNumberId
                Group By UPN.UserId
                ) As PhoneNumbers
    On PhoneNumbers.UserId = Users.UserId

在此解决方案中，对于每个用户和电话号码类型，我从UserPhoneNumbers表中选择最低的主键值（我猜这个列名为UserPhoneNumberId）。

答案 5 :(得分：0)

当有两个相同类型的数字时，您必须定义“first”的含义，然后向连接添加条件，以便只有正确的记录符合条件。这没有其他捷径。

答案 6 :(得分：0)

坚持，只是为了理解这个问题。

你有两张桌子：

  

用户（UserID - ＆gt; x）UserPhones（UserID，PHoneType - ＆gt;电话号码）   和UserID / PhoneType不是唯一的。

首先，不需要临时表：

Select 
 x
from
 Users
inner join 
 (
   Select 
    top 1 y
   from
    FoneTypes
   where
    UserID = users.UseriD
   and phoneType = 'typex'
 ) as PhoneTypex on phonetypex.UserID = users.userID

根据需要添加内部联接。

或者我错过了什么？

答案 7 :(得分：0)

你可以使用GROUP BY：

SELECT  Users.UserID, 
  Users.FirstName, Users.LastName,
  HomePhone, WorkPhone, FaxNumber

FROM Users

LEFT JOIN
 (SELECT UserID, min(PhoneNumber) AS HomePhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Home'
 GROUP BY userID) AS tmpHomePhone
 ON tmpHomePhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, min(PhoneNumber) AS WorkPhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Work'
 GROUP BY userID) AS tmpWorkPhone
 ON tmpWorkPhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, min(PhoneNumber) AS FaxNumber
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Fax'
 GROUP BY userID) AS tmpFaxNumber
 ON tmpFaxNumber.UserID = Users.UserID

代替min（），你也可以使用max（）。

或者你可以通过以下方式在一个小组中完成：

SELECT  Users.UserID, 
  Users.FirstName, Users.LastName,
  max(HomePhone) as HomePhone,
  max(WorkPhone) as WorkPhone,
  max(FaxNumber) as FaxNumber

FROM Users

LEFT JOIN
 (SELECT UserID, PhoneNumber AS HomePhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Home') AS tmpHomePhone
 ON tmpHomePhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, PhoneNumber AS WorkPhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Work') AS tmpWorkPhone
 ON tmpWorkPhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, PhoneNumber AS FaxNumber
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Fax') AS tmpFaxNumber
 ON tmpFaxNumber.UserID = Users.UserID
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