Oracle9i的简化SQL语法

The CROSS JOIN
在Oracle中，CROSS JOIN产生了一个“卡迪尔的产物（Cartesian product）”，就象是在连接两个表格时忘记加入一个WHERE子句一样
select last_name,dept_id
from emp,depts;

在Oracle9i中，我们使用CROSS JOIN 来达到相同的结果
select last_name.dept_id
from emp
CROSS JOIN dept;

NATURAL JOIN

我喜欢NATURAL JOIN的原因在于它能够通过在两个表格中配对的栏目的名头来自动的检查join。它同时还简化了Oracle9i SQL,由于where子句仅仅只能过滤谓语，当然，NATURAL JOIN要求在每一个表格中的栏目的名字相同。很有意思的是，这种特性甚至在没有主要的或是外来的关键词作为参考是也能起作用

Oracle8i,
Select book_title, sum(quantity)
From book, sales
Where book.book_id = sales.book_id
group by book_title;

Oracle9i
Select book_title, sum(quantity)
from book
natural join sales
group by book_title;

USING子句
假如几个栏目有同样的名字，而你又不想用所有的这些栏目来连接的时候，你就可以用USING子句。在USING子句中所列的栏目的句子中不会有任何的修饰词，包括where子句也不会有
Oracle8i
select dept_id, city
from departments, locations
where departments.location_id = location.location_id;

Oracle9i
select department_name, city
from departments
JOIN locations
USING (location_id);

ON子句
ON子句被用于当在两个表格中的栏目名字不搭配时来连接表格。而连接条件就是where子句中的过滤条件
Oracle8i
select department_name, city
from department, location
where department.location_id = location.loc_id;

Oracle9i
select department_name, city
from department d
JOIN location l
ON (d.location_id = l.id);