-used by all commercial db systems
-very simple model
-query with high-level languages: simple yet expressive
-efficient implementations

Database = set named relations (or tables)
Each relation has a set of named attributes (or columns)
Each tuple (or row) has a value for each attribute
Each attribute has a type (or domain)

Schema – structural description of relations in database
Instance – actual contents at given point in time
NULL – unknown or undefined
Key – attribute whose value is unique in each tuple or set of attributes whose combined values are unique

Creating relations (tables) in SQL:
Create Table Student(ID, name, GPA, photo)
Create Table College(name string, state char(2), enrollment integer)