Integrated data means that the database may be thought of as a unification of several otherwise distinct data files, with any redundancy among those files either wholly or partly eliminated.
Consequences of integration are sharing and the idea that any given user will normally be concerned with only a subset of the total database; moreover, different user's subsets will overlap in many different ways i.e. a given database will be perceived by different users in different ways. Also, users may be restricted to certain subsets of data.
Definition of Entity.
An entity is any distinguishable real world object that is to be represented in the database; each entity will have attributes or properties e.g. the entity lecture has the properties place and time . A set of similar entities is known as an entity type.
Network model Overview
A network data structure can be regarded as an extended form of the hierarchic data structure - the principal distinction between the two being that in a hierarchic structure, a child record has exactly one parent whereas in a network structure, a child record can have any number of parents (possibly even zero).
A network database consists of two data sets, a set of records and a set of links, where the record types are made up of fields in the usual way.
Networks are complicated data structures. Operators on network databases are complex, functioning on individual records, and not sets of records. Increased complexity does not mean increased functionality and the network model is no more powerful than the relational model. However, a network-based DBMS can provide good performance because its lack of abstraction means it is closer the the storage structured used though this is at the expense of good user programming. The network model also incorporates certain integrity rules.
System Tables
Information about the database is maintained in the system catalogs. These vary from system to system because the contents of the system catalog is specific to a particular system. The INFORMIX system contains the following tables in it's system catalog.
systables - describes database tables
syscolumns - describes columns in tables
sysindexes - describes indexes in columns
systabauth - identifies table-level privileges
syscolauth - identifies column-level privileges
sysdepend - describes how views depend on tables
syssynonyms - lists synonyms for tables
sysusers - identifies database-level privileges
sysviews - defines views
Who Uses a DBMS
There are three broad classes of users who use a DBMS
