Background:
It is insane to build a table describing every attribute for huge a lot of entities, because there are many spareness in it.
Definition:
1. entity: the things to be described
2. attribute: ways or data to desribe the entity
3. value: the value of attribute
then:
1. a row is a fact
2. attribute and value pair is a fact describing an entity
In detail:
the entity is also like a array, which includes, in the clinical example, name and brief descpriton.
the attribute is cited from attribute definition table as a foreign key.