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。

youdao

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。

youdao

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.

也可以使用系统编目视图来检索关于NOT NULL、惟一、主键、外键和表检查约束的信息。

youdao