一般语法书上把此句归为宾语从句.其实这是一个同位语从句.很多语法书里有很多中式思维.尽管i have no idea=i don't know这只是意义相同而异.
i don't know when they will be back.宾语从句
i have no idea when they will be back.同位语从句.从句是idea的内容.