Foreign Key

[methodsboy]

hey every1, the IT threads have been very quite lately.
does anyone know what a foreign key is? and is it the same as a secondary key?

[ell]

A foreign key is a key in a table that references a field from another table.

E.g. let's say a company takes orders, and they have a Customers table and an Orders table - the Orders table may use the CustomerID field from the Customer table to help uniquely identify an order.

It's separate to a secondary key, which is a different concept that I don't think you need to know for VCE IT.

[methodsboy]

ohhh..ive used that many times..except i had no idea that that's what it was called. LOL!
thanks

[excal]

@ ell - I've never heard of the concept of the secondary key before - is it any different from a candidate key?

For those who are interested, a candidate key is a field that would be suitable as a primary key field in a table but, for whatever reason, is not.

(both of these you won't need to know for IT:A)

[cobby]

@ ell - I've never heard of the concept of the secondary key before - is it any different from a candidate key?

For those who are interested, a candidate key is a field that would be suitable as a primary key field in a table but, for whatever reason, is not.

(both of these you won't need to know for IT:A)

Do you have an example of where a candidate key may be used Excal?

[ell]

@ ell - I've never heard of the concept of the secondary key before - is it any different from a candidate key?

From what I understand, there's secondary indexes - simply a secondary way to access records - which either use candidate keys or a "nonkey with duplicate values" (straight from textbook). The secondary indexes that use candidate keys are called secondary keys, so they are the same thing as a candidate key essentially.

@cobby: You'll start to look at candidate keys when you do normalization of relations into the different normal forms, which you'll learn more about at uni if you decide to do IT.

[excal]

@ ell - I've never heard of the concept of the secondary key before - is it any different from a candidate key?

For those who are interested, a candidate key is a field that would be suitable as a primary key field in a table but, for whatever reason, is not.

(both of these you won't need to know for IT:A)

Do you have an example of where a candidate key may be used Excal?

Candidate keys are generally not used - they're just used to describe fields that are suitable as primary keys, but are not (very simplified explanation). You won't need to look at this unless you  study IT at university...

A more complicated explanation is here: http://en.wikipedia.org/wiki/Candidate_key

(the term 'tuple' in database theory is pretty much a row / record in practice)