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in a relational database, can we have a table without any relation with the other tables?
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Of course. Even you can create a table without fields.
Yes you can. Tables do not have to have any relation to each other. Relations can always be added through the use of foreign keys if you want to add them later.
Yes. The way relations are expressed are with foreign keys. If a table you generate has no Foreign keys, and no foreign keys in other tables point to this table, it has no relationships.
It can still be given a relationship later though so don't worry about shooting yourself in the foot.
I'm creating a company management database and I have a "Company_Expenses" table that I just need for summery reports on the Company's revenues. I can't think of anything this table might need to be connected to so it's just on it's own for now.