文档课题：drop user与drop user cascade的区别.
oracle数据库删除用户有drop user username和drop user username cascade命令，它们有何区别呢？
官方文档：
Dropping a Database User: Example If user Sidney's schema contains no objects, then you can drop sidney by issuing the statement:
DROP USER sidney;
If Sidney's schema contains objects, then you must use the CASCADE clause to drop sidney and the objects:
DROP USER sidney CASCADE;
当用户下没有数据库对象时，可以用drop user username删除用户，但当该用户有对象时，就要用drop user username cascade删除用户.
测试如下：
数据库：oracle 11.2.0.4 64位
系统：centos 7.9 64位
1、drop user测试
SQL> create user liujun identified by liujun;
User created.
SQL> select object_type,count(*) from all_objects where owner='LIUJUN' group by object_type;
no rows selected
SQL> drop user liujun;
User dropped.
说明：用户LIUJUN无数据库对象，drop user命令删除成功.
2、drop user cascade测试
SQL> create user liujun identified by liujun;
User created.
SQL> grant connect,resource,unlimited tablespace to liujun;
Grant succeeded.
SQL> conn liujun/liujun;
Connected.
SQL> select * from user_sys_privs;
USERNAME PRIVILEGE ADM
------------------------------ ---------------------------------------- ---
LIUJUN UNLIMITED TABLESPACE NO
SQL> select * from session_privs;
PRIVILEGE
----------------------------------------
CREATE SESSION
UNLIMITED TABLESPACE
CREATE TABLE
CREATE CLUSTER
CREATE SEQUENCE
CREATE PROCEDURE
CREATE TRIGGER
CREATE TYPE
CREATE OPERATOR
CREATE INDEXTYPE
10 rows selected.
SQL> create table test (id number(6,2),
2 name varchar2(20));
Table created.
SQL> begin
2 for i in 1..1000 loop
3 insert into test values (i,'a' || i);
4 end loop;
5 end;
6 /
PL/SQL procedure successfully completed.
SQL> commit;
Commit complete.
SQL> conn / as sysdba
Connected.
SQL> select object_type,count(*) from all_objects where owner='LIUJUN' group by object_type;
OBJECT_TYPE COUNT(*)
------------------- ----------
TABLE 1
SQL> drop user liujun;
drop user liujun
*
ERROR at line 1:
ORA-01922: CASCADE must be specified to drop 'LIUJUN'
SQL> drop user liujun cascade;
User dropped.
说明：当用户LIUJUN有数据库对象时，删除该用户就需要使用drop user cascade.
