Get One column in join from same table but different database

I have two database.

a. DB_1 b. DB_2

let say, I am currently using DB_1 for my below query

Select top 1 a.mkey,convert(varchar(255), a.ref_date,103) as REF_DATE,  cd.type_desc as DOC_TYPE,  a.doc_no as INWARD_NO, 
                       cr.type_desc as dept_received, e.emp_name as EMP_RECEIVED,    
                       convert(varchar(255), a.doc_date,103) as date,   
                       a.to_user, a.No_of_pages, Ref_No,    
                       e.emp_name as NAME,    
                       coalesce(e.Email_Id_Official, 'test@test.com') EMAILID, a.Party_Name                           
                from inward_doc_tracking_hdr a left join   
                     type_mst_a cd   
                     on a.doc_type = cd.master_mkey left join  
                   type_mst_a cr   
                     on a.dept_received = cr.master_mkey 
                     and cr.type_code='D1'  
                     left join  emp_mst e  
                     on a.emp_received = e.mkey   
                     where a.emp_received is not null and
                        a.mkey = 146

Now what I want is. I want to join the same table from another database(DB_2) how to do that ??

UPDATE

My another db column name is Inward_ref_key and its table name is inward_doc_tracking_hdr