A

• Attributes are pieces of data that describe an entity. For example, a Customer entity may have the following attributes: CustomerNumber, LastName, FirstName, Address, and PhoneNumber.

  Attributes are either atomic or composite. Composite attributes are attributes that could be sub-divided. For example, CustomerName is composite because it can be broken down into atomic parts: CustomerFirstName and CustomerLastName. In a normalized database, we should only have atomic attributes.

  Derived attributes are those that are not actually stored in the database, but could be calculated. e.g. OrderAmount could be derived by multiplying Quantity and Price, so we may choose not to store that value in our database, and instead calculate it when we need it.

D

• Attributes have a data type which describes the kind of data it can hold (e.g. numeric, dates, characters, etc).

• Attributes also have a domain which are the legitimate values it can hold (e.g. “greater than zero”, “exactly 9 characters long”, etc).

E

• An entity is a person, place, thing, or concept. Also called an entity class or entity type. In a normalized database, each table will represent one entity: it will contain data that describes that entity.

  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 as its PK.

F

• A foreign key 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).

P

• Primary keys are unique identifiers for each instance of an entity. Sometimes there isn’t 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.