--判断表是否存在，如果存在则删除declare       num   number; begin       select count(1) into num from all_tables where TABLE_NAME = ‘EMP‘ and OWNER=‘SCOTT‘;       if   num=1   then           execute immediate ‘drop table EMP‘;       end   if; end; / --创建表CREATE TABLE EMP       (EMPNO NUMBER(4) NOT NULL,        ENAME VARCHAR2(10),        JOB VARCHAR2(9),        MGR NUMBER(4),        HIREDATE DATE,        SAL NUMBER(7, 2),        COMM NUMBER(7, 2),        DEPTNO NUMBER(2));