4) Integrity problems:- Data values stored in the database must
satisfy consistency constraints. Problem occurs when
constraints involve several data items from different files.
5) Atomicity problems:- If failure occurs, data must be stored to
constant state that existed prior to failure. For example, if in a
bank account, a person abc is transferring Rs 5000 to the
account of pqr, and abc has withdrawn the money but before it
gets deposited to the pqr’s account, the system failure occurs,
then Rs5000 should be deposited back to abc’s bank account.
6) Concurrent access anomalies:- Many systems allow multiple
users to update data simultaneously. Concurrent updates
should not result in inconsistent data.
7) Security problems:- Not every user of the database system
should be able to access all data. Data base should be
protected from access by unauthorized users.
1.2 DATA INDEPENDENCE
We can define two types of data independence:
1. Logical data independence:
It is the capacity to change the conceptual schema without
having to change external schemas or application programs. We
may change the conceptual schema to expand the database (by
adding a record type or data item), or to reduce the database (by
removing a record type or data item). In the latter case, external
schemas that refer only to the remaining data should not be
affected. Only the view definition and the mappings need be
changed in a DBMS that supports logical data independence.
Application programs that reference the external schema constructs
must work as before, after the conceptual schema undergoes a
logical reorganization. Changes to constraints can be applied also
to the conceptual schema without affecting the external schemas or
2. Physical data independence:
It is the capacity to change the internal schema without
having to change the conceptual (or external) schemas. Changes
to the internal schema may be needed because some physical files
had to be reorganized—for example, by creating additional access
structures—to improve the performance of retrieval or update. If the
same data as before remains in the database, we should not have
to change the conceptual schema. Whenever we have a multiplelevel DBMS, its catalog must be expanded to include information on
how to map requests and data among the various levels. The
DBMS uses additional software to accomplish these mappings by