How to do soft delete and use unique keys?

Question

MySQL

I have a books table that has a unique alias field.
I'm doing a "soft" deletion, now the deleted_at field has been added, and so that deleted books do not interfere with creating new ones with the same alias, I set a unique key to two fields: alias, deleted_at.
Now you can create new entries with the same alias, even if there is such a "deleted" entry.
But now there is a new problem: deleted_at is null by default, so you can create records with the same alias as many times as you want.
How to keep records unique?

Answer 1

I wouldn't use deleted_at as a flag that the entity has been deleted - it's more of a reference.
Would start the is_delete field

Answer 2

Default value deleted_at