SQL中CREATE VIEW 语句不进行判断，若数据库中已经存在的话，则报错，说对象已存在；
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
CREATE OR REPLACE VIEW 语句进行判断，若数据库中已经存在这个名字的视图的话，就替代它，若没有则创建视图；
CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
