Identity (or AutoNumber) is a column that automatically generates numeric values. A start
and increment value can be set, but most DBA leave these at 1. A GUID column also
generates numbers; the value of this cannot be controlled. Identity/GUID columns do not
need to be indexed.
What is DataWarehousing?
Subject-oriented, meaning that the data in the database is organized so that all the data elements relating to the same real-world event or object are linked together;
Time-variant, meaning that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes over time;
Non-volatile, meaning that data in the database is never over-written or deleted, once committed, the data is static, read-only, but retained for future reporting.
Integrated, meaning that the database contains data from most or all of an organization's operational applications, and that this data is made consistent.