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Say you have a table Orders with a foreign key to a Customer Id and you want to add an Order without a Customer Id. (Whether that should be possible is another question.)
Is it bad practice to make the foreign key NULL?
Should you rather work with a link table between Orders and Customers?
Although the relationship is 1-to-n, a link table would make it n-to-n. On the other hand, with a link table, you wouldn't have those NULLs any more.
There won't be a lot of NULLs in the database, because a record with a foreign key to NULL is temporary until a customer for the order is added.
(It isn't an Order and a Customer.)
What about a unassigned Customer to link to?