Each row is then an instance, or occurrence of the type. A query may ask for a subset of the rows of a table, based on the value of an attribute, or a subset of the columns of the table, to show some attributes for all rows, or some combination of each. Data We can identify 4 levels of data that are associated with a database. This information may be stored in the system catalog or data dictionary . Data occurrences or instances.
Data model describes the structures of a DB and the operations that can be done on it.
This is a definition of the DB by intension . These are the values for all the attributes in the table. Conceptual level is a global description of the DB for the entire community of users. Different packages come with different utilities for housekeeping. Backup, e.g. onto tape.