where D.database_id >=5 
    and mirroring_role_desc like CASE WHEN B.mirroring_state is NOT NULL then 'Principal' else null end

Select
d.name,
d.compatibility_level 
FROM  
  sys.database_mirroring M inner join SYS.DATABASES d
  on m.database_id = d.database_id
where D.database_id >=5 
    and mirroring_role_desc like CASE WHEN M.mirroring_state is NOT NULL then 'Principal' 
    else '%'  -- like ANYTHING
    end
ORDER BY D.NAME

Select
d.name,
d.compatibility_level,
m.mirroring_state,
mirroring_role_desc
FROM  
  sys.database_mirroring M inner join SYS.DATABASES d
  on m.database_id = d.database_id
where D.database_id >=5 
    and ISNULL(mirroring_role_desc,'') like CASE WHEN M.mirroring_state is NOT NULL then 'Principal' 
    else '%'  -- like anything (except nulls)
    end
ORDER BY D.NAME