DNS

Domain name server is a system that translates host names into numeric internet addresses.

Varchar vs Varchar2

Both are variable length but varchar stores only 2000 bytes of character of data where as varchar2 stores4000 bytes of character of data. varchar reserves memory for the future usage where in varchar2, string value's length will be stored on disk with the value itself.

Dual table

The dual table is a special one row table presented by default in all oracle database installations. it is suitable for selecting a pseudocolumn such as sysdate or user. the table has a signle varchar2(1) column called dummy, that has a value 'x'. the owner of the dual is "sys", but it can be used by every user.