Consider the folowing CAML query:
<Query>
<Where>
<And>
<Eq>
<FieldRef Name="Field1"/>
<Value Type="Text">Field value 1</Value>
</Eq>
<Eq>
<FieldRef Name="Field2"/>
<Value Type="Text">Field value 2</Value>
</Eq>
<IsNull>
<FieldRef Name="Field3"/>
</IsNull>
</And>
</Where>
SharePoint raises a Microsoft.SharePoint.SoapServer.SoapServerException. What is the problem here?
The “And” element can have only two child elements. So your query could, for example, have this structure, where “And” has “IsNull” plus a nested “And” as child elements.
This article has some good examples: http://msdn.microsoft.com/en-us/library/ms196939.aspx