如果要連接的列爲空，聯接是否仍然有效？

我有這個存儲過程，當我添加連接時，我得不到任何結果。這是因爲用於連接條件的列ForClientID是否爲空？這會影響表格的結果嗎？如果我使用Appointment.ForClientID而不是Client.Name選擇表，則表的其餘部分將加載ForClientID列爲空。在此先感謝，這是一個非常有幫助的社區。如果要連接的列爲空，聯接是否仍然有效？

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

來源

2010-07-14 Gallen

你試過一個LEFT JOIN？ – 2010-07-14 02:15:12

爲什麼不在主查詢中'Appointment'和'asp_Users'之間創建一個連接，在'WHERE'子句中使用'@ ProfessionalName'參數並丟失局部變量'@ ProfessionalID'？ SQL優化器傾向於更喜歡一個大的查詢。 – onedaywhen 2010-07-14 08:02:17

下面是使用OUTER JOIN考慮的Appointment.ForClientID的是NULL -able重新寫入和與INNER JOIN代替局部變量@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;

來源

2010-07-15 09:03:29 onedaywhen

NULL周圍的行爲是不同類型的JOIN之間的主要區別。請閱讀http://en.wikipedia.org/wiki/Join_%28SQL%29，這篇文章雖然是維基百科，但卻是關於這個話題的出色文章。

來源

2010-07-14 02:07:32 Borealid

我用一個左外連接去獲得所需的結果。感謝您指點我正確的方向，有用的閱讀！ – Gallen 2010-07-14 02:13:25

這不是交叉連接，這是SQL-89語法中的標準內連接。我發現笛卡兒產品（即交叉連接）實際上不可能是他想要的。 – Donnie 2010-07-14 02:14:03

如果要連接的列爲空，聯接是否仍然有效？

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