In Oracle 12C release 1 most of SQL and PL/SQL commands are supported in rman:

• no SQL prefix or quotes required
• DESCRIBE is supported

In previous releases

RMAN> sql 'create table test_tbl(id number)';

In Oracle Database 12C

RMAN> create table test_tbl(id number);
RMAN> select * from test_tbl;
RMAN> desc test_tbl

Have a fun 🙂

Tomasz