hibernate 更新多表多字段 报错

 public int copyCount() {
        String hql = "update Content a, ContentCount b set a.viewsDay = b.viewsDay"
                + ",a.commentsDay = b.commentsDay"
                + ",a.downloadsDay = b.downloadsDay"
                + ",a.upsDay = b.upsDay where a.id=b.id";
        return getSession().createQuery(hql).executeUpdate();
    }

执行报错

 antlr.MismatchedTokenException: expecting "set", found ','
    at antlr.Parser.match(Parser.java:211) ~[antlr-2.7.7.jar:na]

mysql直接执行没有问题，怎么这里就报错了

update Comment a set a.content = (select content from Post p where a.id = p.id 这样可以

逗号换成and试试