Properties of Entities

Define Entities, Attributes (ERD Model)

• A relational database table can be
empty (NULL instance)
• Instances Are Unordered
– Order of the rows and columns does not
matter in databases
– Freedom to move the data is limited to
exchanging entire rows or exchanging
entire columns.

 

• Uniqueness
– No two rows can be the same
– Two rows can have the same value for
some attributes, just not all attributes.

• Keys
– Any set of attributes for which all attribute values
are different is called a candidate key
– Pick one and call it the primary key to decide
uniqueness
– Key must distinguish all potential and actual
entities, not just those that happen to be in the
table at a given time
– If no combination of attributes qualify as a
candidate key, assign a unique ID to each entity
• Like a student ID number issued by school.

• Atomic Data
– Values stored for attributes
– Not decomposable into any smaller parts
• Separate fields for street, city, state, postal
code
– “Only atomic data” rule relaxed for certain
types of data
• Dates, times, currency.