Understanding Relational Data Structures and Keys (2)

RELATION

Named two-dimensional table of data consisting of a set of named columns and an arbitrary number of unnamed rows.

Attribute

A named column in a relation.

Record

Each row corresponds to a record that contains data values for a single entity.

Relation Structure

Shorthand notation of relation.

Properties of Relation

Each relation in a database has a unique name, and each entry at the intersection of each row and column is ATOMIC (single value).

Primary Key

An attribute (or combination of attributes) that uniquely identifies each row in a relation.

Composite Key

A primary key that consists of more than one attribute.

Foreign Key

An attribute (possibly composite) that serves as the primary key of another relation.

Entity Integrity

Designed to assure that every relation has a primary key, and that the data values for that primary key are all valid.

Referential Integrity

A rule that maintains consistency among the rows of two relations.

Simple Attribute

Each simple attribute of the entity becomes an attribute of the relation.

Multivalued Attribute

When a regular entity contains a multivalued attribute, two relations are created.

Weak Entity

For each weak entity type, create a new relation and include all of the single-valued attributes.

ORDER

ORDER(OrderNo, CustID, Date, TotalAmount)

Associative Entity

is to create three relations: one for each of the two participating entity types, and the third for the associative entity.

ORDER_DETAILS

ORDER_DETAILS(OrderNo, ItemNo, QtyOrdered, Amount)

Schematic Diagram

CUSTOMER, ITEM, PRICE_HISTORY, ORDER, ORDER_DETAILS.