In one aspect, a method of storing data includes receiving instances of data and storing the instances in an entity instance table, allocating a unique identifier to each instance of data received for storage in the entity instance table, such that each instance can be uniquely referred to, and providing a relationship definition table for storing relationship definitions. Each relationship definition has three data fields. The method also includes receiving three unique identifiers of selected instances of data stored in the entity instance table and storing the three unique identifiers as a relationship definition in the relationship definition table. A relationship is defined by the unique identifiers of exactly three instances of data and each of the three unique identifiers is contained in a respective one of the three data fields of the relationship definition.