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But the database does not know whether this is a one-to-one or one-to-many relationship right?The relationships that I make in an ER-Diagram is only to indicate where it should be foreign keys when making the actual tables?

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If A is a candidate key and B is not then the relationship between A and B is 1 to many.

If both A and B are candidate keys then the relationship is 1 to 1.

This is probably a basic(dumb) question but when having a one-to-one relationship in a database the other table has a foreign key ID(in this example).

And in a one-to-many relationship the table contains many foreign keys.

I do not completely grasp the idea of the relationships, even though I have read many tutorials about this. to the database, they are constructs we have invented to better understand how to design the tables.

The big difference in terms of table structure between one-to-one and one-to-many is that in one-to-one it is possible (but not necessary) to have a bidirectional relationship, meaning table A can have a foreign key into table B, and table B can have a foreign key into the associated record in table A.

And all the time, you want to "save" rows/columns in table not duplicating them you will use one-to-many relation with another table.

Or many-to-many :) Let's assume you have a table with two attributes A and B.

One-to-many relationships associate one record in one table with many records in the other table.

Im having trouble understanding what the actual question is.

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