Here's a list of the operators XPDO supports:
'=',
'!=',
'<',
'<=',
'>',
'>=',
'<=>',
' LIKE ',
' IS NULL',
' IS NOT NULL',
' BETWEEN ',
' IN ',
' IN(',
' NOT(',
' NOT (',
' NOT IN ',
' NOT IN(',
' EXISTS (',
' EXISTS(',
' NOT EXISTS (',
' NOT EXISTS(',
' COALESCE(',
' GREATEST(',
' INTERVAL(',
' LEAST(',
'MATCH(',
'MATCH ('
From this page:
http://www.xpdo.org/api/xpdo/om/xPDOQuery.html
It looks like the documentation for MODx is a bit out of date in some places:
http://rtfm.modx.com/display/xPDO20/xPDOQuery.where
There's a nice link to the XPDOQuery::where() API docs on the page, but a link to the operators page (or even an updated list of operators) would be pretty handy.
On another note, is there a better place to make documentation suggestions, or even a way to update the documentation without posting on the forums?