--DDL(database define )create table t_student(       f_id int,       f_name varchar2(10),       f_sex char(2),       f_age int,       f_hobbys varchar2(100))--刪除表drop table t_student--添加字段alter table t_student add f_join_date date;select * from t_student--修改字段alter table t_student modify(f_hobbys varchar2(200));alter table t_student modify(f_join_date timestamp)--重命名字段alter table t_student rename column f_join_date to f_join--重命名表rename t_student to student--刪除字段alter table student drop column f_age;