Foreign key
A column whose values refer to the primary key of another table, such as the customer ID stored on each order.
A foreign key value with no matching row in the other table is called an orphan. Orphans drop out of an ordinary inner join without any warning, so totals come up short.
Where it comes up
- Codes without a lookupA column holds short codes such as 3, A, or XR, and the table that says what each code means is not in the file.
- The same entity in two systemsOne person or organization exists in two systems under different identifiers, with nothing linking the records.
See also
- Primary keyA column, or set of columns, whose value is unique for every row and identifies it: a customer number, an order ID.
- JoinCombining two tables by matching rows that share a value, such as attaching customer names to orders through a customer ID.
- Data integrityThe assurance that data stays correct and intact as it is stored, moved, and changed, with nothing lost, altered, or left pointing at records that no longer exist.