方法：
1. 删除所有外键约束
2. drop表
alter table Da_pic
drop constraint FK_DA_PIC_REFERENCE_DA_PICSU;
drop table Da_pic cascade constraints;
drop table Da_picsubject cascade constraints;
/==============================================================/
/ Table: Da_pic /
/==============================================================/
create table Da_pic (
ID VARCHAR2(200) not null,no VARCHAR2(200),subjiectid VARCHAR2(200),Name VARCHAR2(400),UploadTime DATE,UserId VARCHAR2(200),picdate DATE,place VARCHAR2(200),people VARCHAR2(200),event VARCHAR2(200),background VARCHAR2(200),cameramen VARCHAR2(200),serial INTEGER,IsEffect VARCHAR2(200),constraint PK_DA_PIC primary key (ID)
);
/==============================================================/
/ Table: Da_picsubject /
/==============================================================/
create table Da_picsubject (
ID VARCHAR2(200) not null,title VARCHAR2(200),year VARCHAR2(200),type VARCHAR2(200),gdtime DATE,iseffect VARCHAR2(200),seiral INTEGER,constraint PK_DA_PICSUBJECT primary key (ID)
);
alter table Da_pic
add constraint FK_DA_PIC_REFERENCE_DA_PICSU foreign key (subjiectid)
references Da_picsubject (ID);
