Query returns no rows in 12c

Why I'm getting no rows selected below?

SQL> select a.con_id,b.con_id  from cdb_data_files a, v$containers b where a.con_id=b.con_id ;

no rows selected

SQL>  select con_id from cdb_data_files;

    CON_ID

----------

         1

         1

         1

         1

         1

         1

         1

         2

         2

         3

         3

         3

         3

13 rows selected.

SQL>  select con_id from v$containers;

    CON_ID

----------

         1

         2

         3

SQL>