您的引用有误:

 myRef = database.getReference("STUDENT").child(child);

在上面的数据快照中将是child，例如id:ur14cs002.因此，它当然将永远不存在，因为dataSnapshot将是子代(提供的ID).

In the above the dataSnapshot will be the child which is this id:ur14cs002 for example. So ofcourse it will never exists, since the dataSnapshot will be the child (which is the id provided).

您需要执行以下操作:-

You need to do this:-

  myRef = database.getReference().child("STUDENT");

这样，引用将在节点STUDENT处，然后orderByChild("id").equalTo(child)将为您提供正确的结果.这里的dataSnapshot将是(STUDENT)节点.

This way the reference is at node STUDENT then orderByChild("id").equalTo(child) will give you the right result. Here the dataSnapshot will be the (STUDENT) node.