of address officially. It was only at the turn of the seasons that his mother reminded him that he had to register, which he did a few days later on December 27, 1994. Although John had a promising future, his story ends tragically. John Doe was accidentally hit by a truck on April 1, 2001. The coroner reported his date of death on the very same day. Using a non-temporal database To store the life of John Doe in a current (non-temporal) database we use a table . (In order to simplify Name is defined as the primary key of Person.) John's father officially reported his birth on April 4, 1975. On this date a Smallville official inserted the following entry in the database: Person(John Doe, Smallville). Note that the date itself is not stored in the database. After graduation, John moves out, but forgets to register his new address. John's entry in the database is not changed until December 27, 1994, when he finally reports it. A Bigtown official updates his address in the database. The Person table now contains Person(John Doe, Bigtown). Note that the information of John living in Smallville has been overwritten, so it is no longer possible to retrieve that information from the database. An official accessing the database on December 28, 1994, would be told that John lives in Bigtown. More technically: if a database administrator ran the query on December 26, 1994, the result would be Smallville. Running the same query 2 days later would result in Bigtown. Until his death, the database would state that he lived in Bigtown. On April 1, 2001, the coroner deletes the John Doe entry from the database. After this, running the above query would return no result at all. Using a single axis: valid time or transaction time Valid time is the time for which a fact is true in the real world. A valid time period may be in the past, span the current time, or occur in the future. For the example above, to record valid time, the Person table has two fields added, Valid-From and Valid-To. These specify the period when a person's address is valid in the real world. On April 4, 1975, John's father registered his son's birth. An official then inserts a new entry into the database stating that John lives in Smallville from April 3. Note that although the