Hi Leon,
yes you are right. This task could not be accomplished with a single query. You have to use two queries.
With the first query you retrieve all accounts which have at least one opportunity
<fetch mapping="logical" distinct="true" version="1.0">
<entity name="account">
<attribute name="accountid" />
<link-entity name="opportunity" from="customerid" to="accountid" />
</entity>
</fetch>
With the second query, you retrieve all accounts which aren't not part of the first result
<fetch mapping="logical" version="1.0">
<entity name="account">
<attribute name="name" />
<filter>
<condition attribute="accountid" operator="not-in">
<value>{accountid 1}</value>
<value>{accountid 2}</value>
</condition>
</filter>
</entity>
</fetch>
I've made only a short test, but this should solve the issue.
