询求SQL语句：如何得到数据库中所有的方案名(Schemas)，以及该方案所属的所有表名？

Question

select * from DBA_TABLESPACES;

Answer 1

SQL> desc dba_objects;
Name           Type          Nullable Default Comments                                                                     
-------------- ------------- -------- ------- ---------------------------------------------------------------------------- 
OWNER          VARCHAR2(30)  Y                Username of the owner of the object                                          
OBJECT_NAME    VARCHAR2(128) Y                Name of the object                                                           
SUBOBJECT_NAME VARCHAR2(30)  Y                Name of the sub-object (for example, partititon)                             
OBJECT_ID      NUMBER        Y                Object number of the object                                                  
DATA_OBJECT_ID NUMBER        Y                Object number of the segment which contains the object                       
OBJECT_TYPE    VARCHAR2(18)  Y                Type of the object                                                           
CREATED        DATE          Y                Timestamp for the creation of the object                                     
LAST_DDL_TIME  DATE          Y                Timestamp for the last DDL change (including GRANT and REVOKE) to the object 
TIMESTAMP      VARCHAR2(19)  Y                Timestamp for the specification of the object                                
STATUS         VARCHAR2(7)   Y                Status of the object                                                         
TEMPORARY      VARCHAR2(1)   Y                Can the current session only see data that it place in this object itself?   
GENERATED      VARCHAR2(1)   Y                Was the name of this object system generated?                                
SECONDARY      VARCHAR2(1)   Y                Is this a secondary object created as part of icreate for domain indexes?    

Answer 2

是不是SELECT * FROM DBA_USERS?那得到的是所有用户，如何得到自定义的用户？不需要系统用户

Answer 3

desc不是sql语句吧？是不是什么工具里面的命令？