如果要连接的列为空，联接是否仍然有效？

我有这个存储过程，当我添加连接时，我得不到任何结果。这是因为用于连接条件的列ForClientID是否为空？这会影响表格的结果吗？如果我使用Appointment.ForClientID而不是Client.Name选择表，则表的其余部分将加载ForClientID列为空。在此先感谢，这是一个非常有帮助的社区。如果要连接的列为空，联接是否仍然有效？

Create procedure GetAppointmentsByProfessionalName(@ProfessionalName varchar(256)) 
as 
    declare @ProfessionalID uniqueidentifier 
    set @ProfessionalID = (select UserId from aspnet_Users where UserName = @ProfessionalName) 

    select Appointment.AppointmentID as 'Appointment ID', 
      Client.Name as 'Client Name', 
      CONVERT(VARCHAR(10),Appointment.ProposedDate,111) as 'Date', 
      CONVERT(CHAR(8), Appointment.ProposedTime, 114)as 'Time', 
      Appointment.ClientDescription as 'Client Notes', 
      Appointment.Confirmed as 'Confirmed Appointment' 
    from Appointment join Client on Client.ClientID = Appointment.ForClientID 
    where Appointment.ForProfessionalID = @ProfessionalID 
go

来源

2010-07-14 Gallen

你试过一个LEFT JOIN？ – 2010-07-14 02:15:12

为什么不在主查询中'Appointment'和'asp_Users'之间创建一个连接，在'WHERE'子句中使用'@ ProfessionalName'参数并丢失局部变量'@ ProfessionalID'？ SQL优化器倾向于更喜欢一个大的查询。 – onedaywhen 2010-07-14 08:02:17

下面是使用OUTER JOIN考虑的Appointment.ForClientID的是NULL -able重新写入和与INNER JOIN代替局部变量@ProfessionalID：

CREATE PROCEDURE GetAppointmentsByProfessionalName 
@ProfessionalName VARCHAR(256) 
AS 
BEGIN 

    SELECT Appointment.AppointmentID AS 'Appointment ID', 
      COALESCE(Client.Name, '{{NO CLIENT}}') AS 'Client NAME', 
      CONVERT(VARCHAR(10),Appointment.ProposedDate,111) AS 'Date', 
      CONVERT(CHAR(8), Appointment.ProposedTime, 114)AS 'Time', 
      Appointment.ClientDescription AS 'Client Notes', 
      Appointment.Confirmed AS 'Confirmed Appointment' 
    FROM Appointment 
      LEFT OUTER JOIN Client 
      ON Client.ClientID = Appointment.ForClientID 
      INNER JOIN aspnet_Users 
      ON aspnet_Users.UserId = Appointment.ForProfessionalID 
    WHERE aspnet_UsersUserName = @ProfessionalName; 

END;

来源

2010-07-15 09:03:29 onedaywhen

NULL周围的行为是不同类型的JOIN之间的主要区别。请阅读http://en.wikipedia.org/wiki/Join_%28SQL%29，这篇文章虽然是维基百科，但却是关于这个话题的出色文章。

来源

2010-07-14 02:07:32 Borealid

我用一个左外连接去获得所需的结果。感谢您指点我正确的方向，有用的阅读！ – Gallen 2010-07-14 02:13:25

这不是交叉连接，这是SQL-89语法中的标准内连接。我发现笛卡儿产品（即交叉连接）实际上不可能是他想要的。 – Donnie 2010-07-14 02:14:03

如果要连接的列为空，联接是否仍然有效？

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