当我们创建一个涉及到很多具有外键约束、视图、触发器、函数等的表的复杂数据库结构时，我们隐式地创建了一张对象之间的依赖关系网。例如，具有一个外键约束的表依赖于它所引用的表。

为了确保整个数据库结构的完整性，PostgreSQL makes sure that you cannot drop objects that other objects still depend on. For example, attempting to drop the products table we had considered in 第 2.4.5 节, with the orders table depending on it, would result in an error message such as this:

DROP TABLE products;
NOTICE:  constraint $1 on table orders depends on table products
ERROR:  Cannot drop table products because other objects depend on it
        Use DROP ... CASCADE to drop the dependent objects too

The error message contains a useful hint: If you don't want to bother deleting all the dependent objects individually, you can run

DROP TABLE products CASCADE;

and all the dependent objects will be removed. In this case, it doesn't remove the orders table, it only removes the foreign key constraint. (If you want to check what DROP ... CASCADE will do, run DROP without CASCADE and read the NOTICE messages.)

PostgreSQL 的所有删除命令都支持指定 CASCADE。当然，可能的依赖的性质随对象的类型而不同。你也可以写 RESTRICT 来代替 CASCADE，以获得默认行为，即限制删除其他对象所依赖的对象。