oracle创建表之前判断表是否存在，如果存在则删除已有表

--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));

摘自: http://www.cnblogs.com/xwdreamer/archive/2012/06/18/2553415.html