1.define constraints as immediate or deferred
　　sql> alter session set constraint[s] = immediate/deferred/default;
　　set constraint[s] constraint_name/all immediate/deferred;
　　2. sql> drop table table_name cascade constraints
　　sql> drop tablespace tablespace_name including contents cascade constraints
　　3. define constraints while create a table
　　sql> create table xay(id number(7) constraint xay_id primary key deferrable
　　sql> using index storage(initial 100k next 100k) tablespace indx);
　　primary key/unique/references table(column)/check
　　4.enable constraints
　 　sql> alter table xay enable novalidate constraint xay_id;
　　 5.enable constraints
　　sql> alter table xay enable validate constraint xay_id;