NULLIF Function

NULLIF Function
In Oracle/PLSQL, the NULLIF function compares expr1 and expr2. If expr1 and expr2 are equal, the NULLIF function returns NULL. Otherwise, it returns expr1.
http://www.techonthenet.com/oracle/functions/nullif.php

NVL Function

NVL Function
In Oracle/PLSQL, the NVL function lets you substitute a value when a null value is encountered.
http://www.techonthenet.com/oracle/functions/nvl.php

The Oracle DUAL table

The Oracle DUAL table
dual is a table which is created by oracle along with the data dictionary. It consists of exactly one column whose name is dummy and one record. The value of that record is X.
http://www.adp-gmbh.ch/ora/misc/dual.html

What are the difference between DDL, DML and DCL commands ?

DDL
Data Definition Language (DDL) statements are used to define the database structure or schema. Some examples:

    CREATE - to create objects in the database
    ALTER - alters the structure of the database
    DROP - delete objects from the database
    TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
    COMMENT - add comments to the data dictionary
    RENAME - rename an object

DML
Data Manipulation Language (DML) statements are used for managing data within schema objects. Some examples:

    SELECT - retrieve data from the a database
    INSERT - insert data into a table
    UPDATE - updates existing data within a table
    DELETE - deletes all records from a table, the space for the records remain
    MERGE - UPSERT operation (insert or update)
    CALL - call a PL/SQL or Java subprogram
    EXPLAIN PLAN - explain access path to data
    LOCK TABLE - control concurrency


DCL
Data Control Language (DCL) statements. Some examples:

    GRANT - gives user's access privileges to database
    REVOKE - withdraw access privileges given with the GRANT command

TCL
Transaction Control (TCL) statements are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.

    COMMIT - save work done
    SAVEPOINT - identify a point in a transaction to which you can later roll back
    ROLLBACK - restore database to original since the last COMMIT
    SET TRANSACTION - Change transaction options like isolation level and what rollback segment to use

http://www.orafaq.com/faq/what_are_the_difference_between_ddl_dml_and_dcl_commands

ICMP Destination Unreachable

ICMP Destination Unreachable
The Destination Unreachable message is an ICMP message which is generated by the host or its inbound gateway to inform the client that the destination is unreachable for some reason. A Destination Unreachable message may be generated as a result of a TCP, UDP or another ICMP transmission. Unreachable TCP ports notably respond with TCP RST rather than a Destination Unreachable type 3 as might be expected.
http://en.wikipedia.org/wiki/ICMP_Destination_Unreachable

IP-Lookup

IP-Lookup
Every machine that is on a TCP/IP network ( a local network, or the Internet ) has a unique Internet Protocol ( IP ) address.
IP-Lookup helps you to find information about your current IP address or any other IP address. It supports both IPv4 and IPv6 addresses.

negative float

Basically negative float is the amount of time the project is behind, as determined by the total time for the tasks on the critical path exceeding the time available for the project.  There is only zero or negative float on the critical path; by definition the critical path doesn't have positive float.  Negative float is something you want to fix.

http://www.projectmanagementquestions.com/2538/when-to-use-a-negative-float