How to use CASE expression in OQL SELECT

I'm trying to return a conditional expression within the SELECT part of an OQL statement:

SELECT (...)

COUNT(ReplyUnread.Content) as RepliesUnread,
CASE RepliesUnread WHEN '0' THEN 'noUnread' ELSE 'hasUnread'

(...)

However, the query cannot be parsed, without any clear error description. How should I use the case statement?