primary key and foreign key

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SQL-Server

database

Is this possible in an sql database?

balance table
pk balanceid
fk payid

payment table
pk payid
fk balanceid

Posted 6-Sep-11 3:33am

wolfsor

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_Zorro_ 6-Sep-11 12:11pm

Just out of curiosity, what would be the point of doing that? I can't picture any scenario where I would need such design.

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CPallini · Answer 1 · 2011-09-06T03:55:00

Solution 1

Yes (if both balanceid in balance table and payid in payment table satisfy primary key requirements).
We call it 'double linked list'. :-)

Posted 6-Sep-11 3:55am

CPallini

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Wendelius 6-Sep-11 11:39am

Good answer, my 5 Just minor note: those key columns must be either primary keys or unique keys :)

Wendelius · Answer 2 · 2011-09-06T05:38:00

Yes this is possible. And as CPallini noted the PK must mean ether primary key or unique key.

However, this may not be the best design. Depending on the situation this may lead to multiple statements, for example first insert to payment, get the new payid and then update the balance table.

If the function of the payid in balance table is just to inform the latest payment, consider adding a time or identity information to payment so that you can always fetch the latest (or second or third...) payment without having to maintain the foreign key.