这是一种方式:

const commentList = this.af.database.list('/eComments', {
      preserveSnapshot: true,
      query: {
        orderByChild: 'sid',
        equalTo: sid
      }
    });
commentList.subscribe(snapshots=>{
    snapshots.forEach(snapshot => {
      snapshot.ref.remove();
    });
})

我不是 AngularFire2 专家，所以可能有更简单/更惯用的方法.

I am not an AngularFire2 expert, so there might be easier/more idiomatic ways.