如何搜索联系人没有组

I have a question about android contacts. I want to query all contacts have not assigned to any group, but I am not sure about the search condition, I mean, sometimes, the GroupMembership.GROUP_ROW_ID is null If the contacts have no group, but sometimes the GroupMembership.GROUP_ROW_ID is -1. Can anyone tell me how to search all contacts have no group?I have a question about android contacts. I wan