如何修复此SQL GROUP BY查询？

Question

我有以下查询：

SELECT 
    dev.DeviceName, Count(dom.DomainID) AS CountOfDomains 
FROM 
    tblDevices dev 
JOIN 
    tblIPNumbers ip ON dev.DeviceName = ip.ServerName 
JOIN 
    tblDomains dom ON dom.IPNumberID = ip.IPNumberID 
WHERE 
    dom.PointerTo=0 
    AND dev.DeviceType='3' 
    AND (dev.[System]='32' OR dev.[System]='33') 
    AND dom.ClosedDate IS NULL AND dev.Active=1 
GROUP BY 
    dev.DeviceName 
ORDER BY 
    Count(dom.DomainID) 

的表格看起来像：

 
tblDomains 
========== 
DomainID  int 
IPNumberID  int 
ClosedDate  datetime 
PointerTo  int 

tblIPNumbers 
============ 
IPNumberID  int 
ServerName  varchar(200) 

tblDevices 
========== 
DeviceID  int 
DeviceName  varchar(200) 
System   varchar(10) 
DeviceType  varchar(10) 
Active   bit 

样本数据：

 
tblDomains: 
=========== 
DomainID: 1234 IPNumberID: 1000 ClosedDate: NULL PointerTo: 0 

tblIPNumbers: 
============= 
IPNumberID: 1000 ServerName: WIN2008-01 

tblDevices: 
=========== 
DeviceID: 1 DeviceName: WIN2008-01  System: 32 Active: 1 DeviceType: 3 

的问题是，如果在tblDomains没有行匹配一个IPNumberID在tblIPNumbers我没有返回行。在这种情况下，我希望查询返回0的单行Count(dom.DomainID) AS CountOfDomains。

我试过LEFT和RIGHT连接的各种组合，它似乎是一个简单的问题，但我今天的SQL-fu很低。

Answer 1

您应该使用LEFT JOIN！而非JOIN秒，也提防，当LEFT JOIN不右表中找到匹配的行，它会为任何领域从右表返回NULL秒。

这意味着您的问题来自您的WHERE子句，该子句以非NULL安全方式引用dom.pointerto，这将有效地使LEFT JOIN无效。

Answer 2

将您的JOIN s更改为LEFT JOIN，它们也会包含不匹配的记录。

Answer 3

尝试使用外连接表tbldomains

SELECT 
    dev.DeviceName, Count(dom.DomainID) AS CountOfDomains 
FROM 
    tblDevices dev 
JOIN 
    tblIPNumbers ip ON dev.DeviceName = ip.ServerName 
outer JOIN 
    tblDomains dom ON dom.IPNumberID = ip.IPNumberID 
WHERE 
    dom.pointerto=0 
    AND dev.devicetype='3' 
    AND (dev.[System]='32' OR dev.[System]='33') 
    AND dom.ClosedDate IS NULL AND dev.active=1 
GROUP BY 
    dev.DeviceName 
ORDER BY 
    Count(dom.DomainID)