SQL Server 查询：使用 JOIN 包含 NULL 值答案

【问题标题】：SQL Server Query: Using JOIN to include NULL valuesSQL Server 查询：使用 JOIN 包含 NULL 值
【发布时间】：2014-06-15 08:10:34
【问题描述】：

我需要以下 SQL Server 查询的帮助，其中列 a.TAProfileID 和 c.CountryCode 在数据库中具有“NULL”值。

我希望我的 JOIN 语句在它们存在的地方返回“NULL”值。

SELECT 
a.ReservationStayID AS 'Reservation Id',
a.PMSConfirmationNumber as 'PMS No',
a.CreatedOn AS 'Date Created',
a.ArrivalDate AS 'Date of Arrival',
a.DepartureDate AS 'Date of Departure',
a.TAProfileID AS 'TA Id',
a.StatusCode AS 'Status',
b.PropertyCode AS 'Hotel',
c.Name AS 'Travel Agency',
c.CountryCode AS 'Market Code',
d.CountryName AS 'Mkt'

FROM ReservationStay a

inner JOIN GuestStaySummary b ON a.ReservationStayID = b.ReservationStayID
inner JOIN TravelAgency c ON a.TAProfileID = c.TravelAgencyID
inner JOIN Market d ON c.CountryCode = d.CountryCode

【问题讨论】：

标签： sql join null sql-server-2012

【解决方案1】：

为了返回或产生NULL 值，您必须使用LEFT JOINs。

所以，您的查询应该是这样的：

SELECT 
     a.ReservationStayID AS 'Reservation Id'
    ,a.PMSConfirmationNumber AS 'PMS No'
    ,a.CreatedOn AS 'Date Created'
    ,a.ArrivalDate AS 'Date of Arrival'
    ,a.DepartureDate AS 'Date of Departure'
    ,a.TAProfileID AS 'TA Id'
    ,a.StatusCode AS 'Status'
    ,b.PropertyCode AS 'Hotel'
    ,c.NAME AS 'Travel Agency'
    ,c.CountryCode AS 'Market Code'
    ,d.CountryName AS 'Mkt'
FROM ReservationStay a
    INNER JOIN GuestStaySummary b ON a.ReservationStayID = b.ReservationStayID
    LEFT JOIN TravelAgency c ON a.TAProfileID = c.TravelAgencyID
    LEFT JOIN Market d ON c.CountryCode = d.CountryCode

【讨论】：

工作出色！ :-)
外连接不使用现有的 NULL 值进行匹配。当匹配不存在时，它们会产生 NULL 值。