For most of the last two decades, data modeling has centered around the relationships among various entities. A person has one account but one or many phone numbers. That same person has one or more addresses (such as a home and work address). A person can belong to one or more groups, which can in turn contain many people.
nge over time.
We modeled these relationships using foreign keys and join tables, and we built queries by joining multiple tables together to produce the desired result. But in recent years we have begun to introduce another dimension to our data: time. Now we're interested in more than just how entities are connected, but also how their relationships change over time.
For example, while we previously were concerned only about a set of fixed locations associated with a person, we now have mobile phones with GPS radios in pockets and purses all over the world. This makes it possible to produce a timeline of a person's movements, marrying time...
