So, recently I was doing some work with SharePoint Web Services, getting possible values for Lookup fields. One of the functions I was using was GetListItems(), to get the values that the lookup field could take.

The method’s signature is:

Lists.GetListItems( [List], [ViewName], [Query], [ViewFields], [RowLimit], [QueryOptions], [WebID]);

I’m not going to talk about most of these parameters – you can read about them here. For my query, most of these parameters were Null (i.e. they were unused). However, the [List] parameter interested me. It’s the ‘Name’ of the list, though I prefer using a GUID (a unique ID) for the list.

The lookup column I was using was defined in a different site to the one where I was making the GetListItems call. I’d pass the GUID, and get the correct values back. My call actually was:

XmlNode resultNode = _lists.GetListItems( listGuid, null, null, viewFields, null, null, null);

So, this query was finding the correct list elsewhere in the site collection based only on the GUID. Interesting – I’m not passing a WebID, and the list is outside the site I’m currently using the web services of. This set me thinking – clearly the call was selecting by list GUID against a table of all ListItems in the site collection.

I was a little surprised – then it hit me – the Content Query Web Part also does that. But how?

Well, a bit of searching later and I found the SPSiteDataQuery object. The object is used for queries against multiple lists in multiple sites. You can specify the types of list to query, values to filter, order by, display, etc.. I’m pretty sure that this is what the Content Query Web Part uses.

Neat! Now I just need to find a reason to use it!