Hibernate的delete()方法
数据库的外键约束有级联删除，级联更新，set null三种。。。

hibernate的cascade="delete"可以忽略数据库外键约束。。。

因为

Hibernate: update book set type_id=null where type_id=1//先将外键设为nullHibernate: delete from book where id=1Hibernate: delete from book where id=2Hibernate: delete from book where id=3Hibernate: delete from book where id=4Hibernate: delete from book_type where id=1

所以在create table B的时候，从表B的外键列要可以为null