SQL> delete user itp2;
delete user itp2
       *
第 1 行出现错误:
ORA-00903: 表名无效

SQL> drop user itp2;
drop user itp2
*
第 1 行出现错误:
ORA-01940: 无法删除当前已连接的用户

SQL> drop user itp2;

用户已删除。

SQL> drop user itp;
drop user itp
*
第 1 行出现错误:
ORA-01922: 必须指定 CASCADE 以删除 'ITP'

SQL> drop user itp cascade;

用户已删除。

SQL>

drop user ； 仅仅是删除用户，

drop user ×× cascade ；会删除此用户名下的所有表和视图。

user
Specify the user to be dropped. Oracle Database does not drop users whose schemas 
contain objects unless you specify CASCADE or unless you first explicitly drop the 
user's objects.

CASCADE 
Specify CASCADE to drop all objects in the user's schema before dropping the user. You 
must specify this clause to drop a user whose schema contains any objects.

使用cascade参数可以删除该用户的全部objects。要说明的如下：

1 If the user's schema contains tables, then Oracle Database drops the tables and 
automatically drops any referential integrity constraints on tables in other schemas 
that refer to primary and unique keys on these tables.

如果用户的schema中有表，则在删除表的时候自动删除与该表相关的主键和外键。

2 If this clause results in tables being dropped, then the database also drops all 
domain indexes created on columns of those tables and invokes appropriate drop 
routines. 
如果用户的schema中有表，则在删除表的时候自动删除与该表相关的索引。

3 Oracle Database invalidates, but does not drop, the following objects in other 
schemas:
删除用户时，下列在其他用户中的objects不会被删除，只会被置为无效

1 Views or synonyms for objects in the dropped user's schema
视图，同义词

2 Stored procedures, functions, or packages that query objects in the dropped 
user's schema
存储过程，函数，包

4 Oracle Database does not drop materialized views in other schemas that are based 
on tables in the dropped user's schema. However, because the base tables no 
longer exist, the materialized views in the other schemas can no longer be 
refreshed.
其他用户建立的基于被删除用户的物化视图不会被删除，只是不能在刷新了。

5 Oracle Database drops all triggers in the user's schema.
用户模式下的所有触发器全部被删除

6 Oracle Database does not drop roles created by the user. 
被删除用户建立的其他用户不会被删除