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 DELETESETNULL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。
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 DELETESETNULL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。
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