Data in a database has a definite hierarchy. One task involved with setting up a database is
data definition (the process of describing data to be entered into a DBMS). The data definitions
are used to create a table structure for each table. The table structure contains a description of
the data to be entered into the table (field name, data type, and other properties). The data
dictionary contains information (called metadata) about all data in the database application.
Different types of database may relate their entities differently. Common types of entity
relationships include one-to-one, one-to-many, and many-to-many. A characteristic of an entity
is called an attribute. Entities typically become database tables; attributes typically become
table fields.