How to do a group by in sharepoint 2013 discussion Board?

I have sharepoint 2013 on prime and sharepoint 2013 online.

I created a discussion board and added a new column named "Dept".

I want to customize the subject view so that all threads are grouped by "Dept".

I tried it but nothing showing up instead of this " By...|" on the default view page.

Please kindly give your recommdations on how to do this.

Thanks in advance

Henry