A
- Attributes are pieces of data that describe an entity. For example, a Customer entity may have the following attributes: customer number, last name, first name, address, and phone number.
Attributes are either atomic or composite. Composite attributes are attributes that could be sub-divided. For example, CustomerName is composite, but CustomerFirstName and CustomerLastName would be atomic.
Derived attributes are those that are not actually stored in the database, but could be calculated, like OrderAmount could be derived by multiplying Quantity and Price.
C
- Cardinality refers to the number of instances of one entity that are associated with an instance of another entity.
In general, there may be one-to-one, one-to-many, or many-to-many relationships.
D
- Attributes have a data type which describes the kind of data it can hold (e.g. numeric).
- Attributes also have a domain which are the legitimate values it can hold (e.g., “greater than zero”).
E
- An entity is a person, place, thing, or concept. Also called an entity class or entity type.
Associative entities are entities which connect two other entities who have a many:many relationship, and they usually contain the primary key attributes of both the other entities.
F
- A Foreign Key (FK) is how we define relationships between entities. A foreign key is the primary key of another entity (the parent) that appears as an attribute of this entity (the child).
O
- Ordinality relates to whether an entity is mandatory or optional with another associated entity.
P
- Primary Keys are unique identifiers for each instance of an entity. Sometimes there is not a naturally occuring identifier, so we make one up. These are called technical keys, and are created in SQL using
IDENTITY.
A combination of 2 or more attributes acting as the primary key is called a concatenated key or composite key.
R
- A relationship is the association that describes the interaction between entities.