oracle中设置表中主键字段自增

create table xw_table(xh number(10) primary key not null,xm varchar2(10),sex char(2) check(sex in('男','女')));

create sequence xw_sequence
minvalue     1
nomaxvalue
start with   1
increment by 1
nocycle

create trigger xw_table_trigger before insert on xw_table for each row when(new.xh is null)
begin
select xw_sequence.nextval into:new.xh from dual;
end;