What is a foreign key?

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forest8

Hi

Can someone explain to me what is a foreign key? When should I use it? How
are they work in a database?

Thanks
 
forest8 said:
Can someone explain to me what is a foreign key? When should I use it? How
are they work in a database?
Click to expand...


A foreign key is a field in one table that identifies a specific record in
another table. Usually, the foreign key field holds a value that is the
primary key of a record in the other table. Thus, the foreign key field
serves to link related records in the two tables.

For example, you might have a table of Customers and a table of Orders.
Every order must be for a particular customer. These tables might contain
fields like these:

Customers
----------------
CustomerID (primary key; uniquely identifies the customer)
CustomerName
CustomerAddress

Orders
-----------------
OrderID (primary key; uniquely identifies the order)
OrderDate
OrderTotal
ShipDate
CustomerID (foreign key; identifies the customer
who placed this order)

The foreign key field CustomerID in the Orders table relates each order to
the specific customer who placed the order.

In a query, joining two tables on the key field(s) that relate them allows
the query to extract all the relevant information from both tables.
 
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