When ON delete set NULL is active, when a record is deleted from the parent table, all records in the child table with matching foreign key values are set to NULL.
当ON DELETE SET NULL规则被激活时，如果一条记录从父表中删除，子表中的所有记录以及匹配的外键值都被设置为NULL。
2
Any value except Null can serve as a key; any value can be assigned to a slot.
除Null以外的任何值都可以充当一个键；任何值都可以被分配给一个slot。
3
You can also use the system catalog views to retrieve information about NOT NULL, unique, primary key, foreign key, and table check constraints. For example.