show all databases sqlplus
1) To view database
select * from v$database;
2) To view instance
select * from v$instance;
3) To view all users
select * from all_users;
4) To view table and columns for a particular user
select tc.table_name Table_name
,tc.column_id Column_id
,lower(tc.column_name) Column_name
,lower(tc.data_type) Data_type
,nvl(tc.data_precision,tc.data_length) Length
,lower(tc.data_scale) Data_scale
,tc.nullable nullable
FROM all_tab_columns tc
,all_tables t
WHERE tc.table_name = t.table_name;