select table_name from user_tables; //當前用戶擁有的表   
select table_name from all_tables; //所有用戶的表 
select table_name from dba_tables; //包括系統表 

select * from user_tab_comments; //當前用戶擁有表的注釋
select * from dba_tab_comments; //全部用戶表的注釋
select * from all_tab_comments; //全部用戶表的注釋
dba_tab_comments, all_tab_comments 比 user_tab_comments 要多一個ower列 

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