select table_name from user_tables; //当前用户拥有的表   
select table_name from all_tables; //所有用户的表 
select table_name from dba_tables; //包括系统表 

select * from user_tab_comments; //当前用户拥有表的注释
select * from dba_tab_comments; //全部用户表的注释
select * from all_tab_comments; //全部用户表的注释
dba_tab_comments, all_tab_comments 比 user_tab_comments 要多一个ower列 

select * from user_col_comments;
select * from dba_col_comments;
select * from all_col_comments;