Design entities from the nouns used in a data definition.
Entities become the tables later when the database is actually designed.
Entities relate to one another to provide a tie between tables. This relationship is usually one-to-many, but could be one-to-one when denormalized.
Many-to-many relationships are defined by using three tablesthe two tables being related, and each connected to a junction table in the middle by a one-to-many relationship.
Each entity should have a primary key that defines each record of the entity as a unique data element. This primary key is used when developing relationships between entities.
Foreign keys in a related table reference the main table's primary key. In this manner, a record cannot be added to a subsidiary table if a matching key is not found in the primary table.
Denormalization can be applied to a structure to improve performance and reduce server overhead.
XML data definitions are defined within the XML schema and developed from an ER model, similar to a database.
Schemas (XSD) are preferred over Document Type Definitions, as they use a standard XML mechanism for coding and are more flexible, allowing for more precise data definition structure.
XML data is composed of elements and attributes; typically, elements are the data records, and attributes serve as descriptors of the data record containing them.
First normal form is avoiding "sets as attributes" through repeating fields (for example, "Phone1," "Phone2") or inserting multiple pieces of data into a single field.
Second normal form conforms to first normal form, ensures that non-key fields are dependent on the entire primary key, and ensures that redundant data is designed out. (For example, the street address of a commonly used vendor should not appear in the table multiple times.)
Third normal form conforms to second normal form and ensures that no fields are dependent on non-primary key fields. (For example, if a State field exists in the Customers table, there should not also be a StateSalesTax field in the same table that is dependent on the State field.)
