Question

I have the following query:

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)

The tables look like:

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

Sample Data:

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

The problem is that if there are no rows in tblDomains that match an IPNumberID in tblIPNumbers I get no rows returned. I’d like the query to return a single row of 0 for Count(dom.DomainID) AS CountOfDomains in this case.

I’ve tried various combinations of LEFT and RIGHT joins and it seems like a simple problem but my SQL-fu is low today.

Answer 1

You should use LEFT JOINs instead of JOINs, and also beware that when a LEFT JOIN doesn’t find a matching row in the right table, it’ll return NULLs for any field from that right table.

That means you issue comes from your WHERE clause that references dom.pointerto in a non-NULL-safe way, which will effectively void the benefits of the LEFT JOIN.