REVERSE函数不能直接赋值

[ ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Thu May 12 22:26:47 2011

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select reverse('abc') from dual;

REV
---
cba

SQL> select reverse('abc') from dual;

REV
---
cba

 

SQL> declare
  2    v_tmp varchar2(20);
  3  begin
  4    v_tmp:=reverse('abc');
  5    dbms_output.put_line(v_tmp);
  6  end;
  7  /
  v_tmp:=reverse('abc');
         *
ERROR at line 4:
ORA-06550: line 4, column 10:
PLS-00201: identifier 'REVERSE' must be declared
ORA-06550: line 4, column 3:
PL/SQL: Statement ignored

SQL> declare
  2    v_tmp varchar2(20);
  3  begin
  4    select reverse('abc') into v_tmp from dual;
  5    dbms_output.put_line(v_tmp);
  6  end;
  7  /
cba

PL/SQL procedure successfully completed.

以前还真不清楚这个函数不可以直接赋值，简单记录一下。