Getting Checked out Files
<Where> <IsNotNull> <FieldRef Name='CheckoutUser' /> </IsNotNull> </Where>
or
<Where> <Eq> <FieldRef Name='_Level' /> <Value Type='Integer'>255</Value> </Eq> </Where>
Getting Published Files
<Where> <Eq> <FieldRef Name='_Level' /> <Value Type='Integer'>1</Value> </Eq> </Where>
Getting Checked-in Files
<Where> <Eq> <FieldRef Name='_Level' /> <Value Type='Integer'>2</Value> </Eq> </Where>
Getting Files Only (i.e. excluding Folders)
<Where> <Eq> <FieldRef Name='FSObjType' /> <Value Type='Integer'>0</Value> </Eq> </Where>
Getting Folders Only (including Document Sets)
<Where> <Eq> <FieldRef Name='FSObjType' /> <Value Type='Integer'>1</Value> </Eq> </Where>
Comparing URLs
<Where> <Eq> <FieldRef Name='WorkflowLink' /> <Value Type='URL'>/subsite/listname/item-url.docx</Value> </Eq> </Where>
Note that you must use a server relative URL when querying against a URL field.
Querying Content Type ID
<Where> <Eq> <FieldRef Name='ContentTypeId' /> <Value Type='ContentTypeId'>0x0120D52000058D5968BC2445489FC509B9E038862900A88F92769A306640872EDF8D34E64427</Value> </Eq> </Where>
Querying Current User
<Where> <Eq> <FieldRef Name="UserField"></FieldRef> <Value Type="User"><UserID /></Value> </Eq> </Where>
or
<Where> <Eq> <FieldRef Name="UserField"></FieldRef> <Value Type="Integer"><UserID /></Value> </Eq> </Where>
Querying User Based on ID
<Where> <Eq> <FieldRef Name="UserField" LookupId="TRUE"></FieldRef> <Value Type="User">11</Value> </Eq> </Where>
or
<Where> <Eq> <FieldRef Name="UserField" LookupId="TRUE"></FieldRef> <Value Type="Integer">11</Value> </Eq> </Where>