What is difference between varchar and varchar2?

Varchar means fixed length char, varchar2 means variable length char.
 

Difference between oracle8i and oracle9i?

a. The biggest difference between Oracle8i and Oracle9i is that Oracle9i lets you resize the SGA memory areas dynamically i.e., the Database Buffer Cache DB_CACHE_SIZE or the SHARED_POOL_SIZE, etc., can be resized when the database is up and running.  The same is not possible with Oracle8i.  You can get more information from OTN.

b. Oracle 9i is an enhanced version of the Oracle 8i.  Oracle 8i does not provided time stamping facility, which is provided in Oracle 9i. 

c. You can drop a column in oracle 9i using alter table command where as this is not available in oracle 8i.

d. Database performance ease of management scalability security availability Windows 2000 integration and application areas: Internet content management commerce integration packaged applications Business Intelligence native compilation of PL/SQL reduction in latch contention (SGA locks) and I/O improvements better Java performance (better compilation, improved garbage collection) distributed database performance enhancements enhanced 3-tier security (integration with   LDAP...) improved hosting security (through use of virtual private   databases), fine-grained auditing and single sign-on improved user   security (more password management features, etc.)  Ability to encrypt stored data row-level access control (Oracle Label Security).

e. Listed below are some of the features of 9i:
1) U has automatic undo management.
2) U has automatic segment space management
3) U got these locally managed tablespaces which is better than the dictionary managed tablespaces as u can avoid fragmentation with locally managed tablespaces.
4) U has a new dbms_metadata package to extract object definitions from database.
5) Multi table insert is possible with 9i
6) Importantly u has this spfile in 9i.
7) Also u has dynamic memory management facility with 9i.
8) U has tuning advisories with 9i.
9) U has multi block size parameter with 9i and many more features like these.
 

What is Pro*C? What is OCI?

Pro *C
The Pro* c/C++ precompiler takes the SQL statement that embedded in C/C++ code convert into standard C/C++ code. When successfully precompile this code, the result is a C or C++ program that we compile and used to build the application that access the Oracle Application.

OCI: - OCI refer to Oracle Call interface is set of Low Level API (Application Program Interface Call) used to interact with Oracle Database. By OCI one can use the operation such as Logon, Fatch, parse, execute etc. Generally these are written in C/C++.
 

What is the difference between "NULL in C" and "NULL in Oracle?

The NULL in C treated as Zero or void. but in SQL NULL value is Non or blank represented it can't manipulated.
 

In Oracle varchar2 takes dynamic space for storage then why char is still in oracle?

The major difference between varchar2 and char is fixed length and variable length. Varchar2 have variable length mean if we declare as 20 space and its use only 5 space the memory assign only 5. But in char takes daclare space while use any number space less than declare.

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