[chaley]

For completeness...

Taken from calibre's search documentation:

Yes/no custom columns are searchable. Searching for false, empty, or blank will find all books with undefined values in the column. Searching for true will find all books that do not have undefined values in the column. Searching for yes or checked will find all books with Yes in the column. Searching for no or unchecked will find all books with No in the column. Note that the words yes, no, blank, empty, checked and unchecked are translated; you must use the current language’s equivalent word. The words true and false and the special values _yes and _no are not translated.