data was inserted on the fourth, the database states that the information is valid since the third. The official does not yet know if or when John will move to another place, so the Valid-To field is set to infinity (∞). The entry in the database is: Person(John Doe, Smallville, 3-Apr-1975, ∞). On December 27, 1994, John reports his new address in Bigtown where he has been living since August 26, 1994. A new database entry is made to record this fact: Person(John Doe, Bigtown, 26-Aug-1994, ∞). The original entry Person (John Doe, Smallville, 3-Apr-1975, ∞) is not deleted, but has the Valid-To attribute updated to reflect that it is now known that John stopped living in Smallville on August 26, 1994. The database now contains two entries for John Doe Person(John Doe, Smallville, 3-Apr-1975, 26-Aug-1994). Person(John Doe, Bigtown, 26-Aug-1994, ∞). When John dies his current entry in the database is updated stating that John does not live in Bigtown any longer. The database now looks like this Person(John Doe, Smallville, 3-Apr-1975, 26-Aug-1994). Person(John Doe, Bigtown, 26-Aug-1994, 1-Apr-2001). Using two axes: valid time and transaction time Transaction time records the time period during which a database entry is accepted as correct. This enables queries that show the state of the database at a given time. Transaction time periods can only occur in the past or up to the current time. In a transaction time table, records are never deleted. Only new records can be inserted, and existing ones updated by setting their transaction end time to show that they are no longer current. To enable transaction time in the example above, two more fields are added to the Person table: Transaction-From and Transaction-To. Transaction-From is the time a transaction was made, and Transaction-To is the time that the transaction was superseded (which may be infinity if it has not yet been superseded). This makes the table into a bitemporal table. What happens if the person's address as stored in the database is incorrect? Suppose an official accidentally entered the wrong address or date? Or, suppose the person lied about their address for some reason. Upon discovery of the error, the officials update the database to correct the information recorded. For example, from 1-Jun-1995 to 3-Sep-2000, John Doe moved to Beachy. But to avoid paying Beachy's exorbitant residence tax, he never reported it to the authorities. Later during a tax investigation,