不同数据库中drop a table if it exists的不同：
In MySQL it is pretty easy to drop a table if it exists already. In Oracle and Microsoft’s SQL Server it is a little more complicated. Today I want to present you the solutions for these two DBMS’.
MySQL:
DROP TABLE IF EXISTS [table_name]
Oracle:
BEGIN
EXECUTE IMMEDIATE 'DROP TABLE [table_name]';
EXCEPTION WHEN OTHERS THEN NULL;
END;
SQL Server:
IF EXISTS (
SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME = '[table_name]')
DROP TABLE [table_name]
ORACLE中先判断表是否存在再新建表一例：