Oracle

timestamp类型
select to_date(time,'yyyy-mm-dd hh24:mi:ss') from table_name;

DATE类型
SELECT TO_DATE(TO_CHAR(time,'MM/DD/YYYY HH24:MI:SS'), 'MM/DD/YYYY HH24:MI:SS') FROM table_name;

field=字段名

alter table table_name add field varchar2(2) ;   //添加字段

comment on column table_name.field is '注释';  //添加注释

alter table table_name modify field default 3;    //设置默认值;