是否可以使用Cascade Delete功能，是可用的外键
是否可以声明用作外键但可能为null的表列？
我有一个表消息，其中包含user_uuid作为对user_profiles表的外键参考。
SELECT * FROM messages;
id user_uuid message
-- --------- -------
1 f52acab5-6115-4a09-ad81-eac662292968 Hello user john smith
2 null Hello all
我想为广播消息允许nulls，但是如果指定了User_uuid，我想确保使用USER_PROFILE表验证它，并在用户删除时执行用户消息的级联删除。 是否有一种方法可以通过数据库手段来做到这一点？
ps我知道制作0000000-0000-0000-0000-0000000000的解决方案，桌子中的伪造记录并引用它。但是有更多文明的方式吗？
Is it possible to declare table column that is used as foreign key, but can be null?
I have a table messages that contains user_uuid as foreign key reference to user_profiles table.
SELECT * FROM messages;
id user_uuid message
-- --------- -------
1 f52acab5-6115-4a09-ad81-eac662292968 Hello user john smith
2 null Hello all
I want to allow nulls for broadcast messages, but if user_uuid is specified, i want to make sure it is validated against user_profile table, and also to perform cascade deletion of user messages in case when user gets deleted.
Is there a way to do so by just database means?
p.s. I know the solution of making 0000000-0000-0000-0000-0000000000 fake record in a table and referencing it. But is there more civilized way?
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您已经正确执行了。如果外键列为null，则该行不会执行参考完整性。
You are already doing it correctly. If the foreign key column is NULL, referential integrity is not enforced for that row.
这是可能的。只需检查以下线程：
postgresql：efirtic键
=“ https://stackoverflow.com/questions/28206232/28206232/can-foreign-key-references-contain-null-values-in-postgresql”>可以外键参考可以包含null值吗？
级联删除也应起作用，因为最后列是有效的参考。
This is possible. Just check following threads:
PostgreSQL: NULL value in foreign key column
Can foreign key references contain NULL values in PostgreSQL?
The cascade deletion should also work, because at the end the column is a valid reference.