SUM function - NULL problem

I have a problem regarding to a SUM function in sql that gives NULLs, thus brakes all the structure of the table: (x-month,y-cities,value-Nettotal) but i get wrongly arranged values according to a y-cities, because there are no free spaces, i mean 0 values, here is the screenshot to make it more clear:

and the link to a question i asked before but couldnt get a clear answer: group by cities

So what I and Dave DuPlantis have wrote so far is:

the query:

<cfquery name="GET_SALES_TOTAL" datasource="#dsn#">
    SELECT SUM(COALESCE(NETTOTAL,0)) NETTOTAL,
           SC.CITY_ID,
           SC.CITY_NAME,
           M.INVOICE_MONTH
    FROM SETUP_CITY SC 
        LEFT OUTER JOIN COMPANY C 
            ON SC.CITY_ID = C.CITY 
                LEFT OUTER JOIN #DSN2_ALIAS#.INVOICE I 
                    ON C.COMPANY_ID = I.COMPANY_ID
        , 
        (
            SELECT DISTINCT DATEPART(MM,INVOICE_DATE) INVOICE_MONTH 
            FROM #DSN2_ALIAS#.INVOICE
        ) M
    WHERE PURCHASE_SALES = 1 
    AND DATEPART(MM,I.INVOICE_DATE) = M.INVOICE_MONTH 
    AND SC.COUNTRY_ID=1
    GROUP BY M.INVOICE_MONTH,
             SC.CITY_ID,
             SC.CITY_NAME
    ORDER BY M.INVOICE_MONTH,
             SC.CITY_ID,
             SC.CITY_NAME
</cfquery>

and table:

<table cellpadding="3" cellspacing="1" class="color-border">
    <tr class="color-header">
        <td class="txtbold" nowrap width="100">Aylar / Sehirler</td>
        <cfoutput query="GET_SALES_GRAND_TOTAL" group="city_id">
            <td class="txtbold">#city_name#</td>
        </cfoutput>
    </tr>
    <cfoutput query="GET_SALES_TOTAL" group="invoice_month"><!--- months first --->
        <tr class="color-row"><!--- month-specific stuff goes here --->
            <td class="txtbold">
                #invoice_month#                         
            </td>
            <cfoutput group="city_id"><!--- city-specific stuff --->
                <td>
                    #tlformat(nettotal,2)#<!--- format NETTOTAL however you want here --->
                </td>
            </cfoutput>
        </tr>
    </cfoutput>
</table>

Does anyone have an idea to solve this kind of problem?! Thank you everyone for help!

As Leigh observes, you need the same number of months for each city for the output to populate as required - by linking invoice to sub-query M in the WHERE clause, you have turned it into an inner join. Amending the query to something like the following should work:

<cfquery name="GET_SALES_TOTAL" datasource="#dsn#">
SELECT SUM(COALESCE(NETTOTAL,0)) NETTOTAL,
       SC.CITY_ID,
       SC.CITY_NAME,
       M.INVOICE_MONTH
FROM SETUP_CITY SC 
    LEFT OUTER JOIN COMPANY C 
        ON SC.CITY_ID = C.CITY 
    CROSS JOIN  
    (   SELECT DISTINCT DATEPART(MM,INVOICE_DATE) INVOICE_MONTH 
        FROM #DSN2_ALIAS#.INVOICE
    ) M
            LEFT OUTER JOIN #DSN2_ALIAS#.INVOICE I 
                ON C.COMPANY_ID = I.COMPANY_ID
               AND DATEPART(MM,I.INVOICE_DATE) = M.INVOICE_MONTH 
WHERE PURCHASE_SALES = 1 
AND SC.COUNTRY_ID=1
GROUP BY M.INVOICE_MONTH,
         SC.CITY_ID,
         SC.CITY_NAME
ORDER BY M.INVOICE_MONTH,
         SC.CITY_ID,
         SC.CITY_NAME
</cfquery>