As specified in the XmlApi documentation:
You can OR two expresions with a comma (,), and AND two expressions with a plus (+).
GET /databases/1/items?output=xml&query=’daap.songartist:Morrissey’,’daap.songartist:The Smiths’
This would return all songs performed by The Smiths or by Morrissey.
GET /databases/1/items?output=xml&query=’daap.songformat:mp3’+’daap.songbitrate+128′
This would get all mp3s with a bitrate higher than 128Kb.
However, when trying to perform a query with the AND (+) operator, it tosses an error and returns ALL of the items in the library.
LOG OUTPUT:
2009-06-01 12:09:44 (b4cffb90): Request: GET /databases/1/items?output=xml&query=’daap.songartist:Korn’+’daap.songalbum:Greatest%20Hits’ HTTP/1.1
2009-06-01 12:09:44 (b4cffb90): Ignoring bad query (‘daap.songartist:Korn’ ‘daap.songalbum:Greatest Hits’): Offset 24: Expecting end of statement
In contrast, the OR (-) operator functions correctly as expected.