Sams Teach Yourself SQL in 21 Days, Fourth Edition

Q&A

Q1:	If the data within my table is already in sorted order, why should I use an index on that table?
A1:	An index still gives you a performance benefit by looking quickly through key values in a tree. The index can locate records faster than a direct access search through each record within your database. Remember—the SQL query processor doesn't necessarily know that your data is in sorted order.
Q2:	Can I create an index that contains fields from multiple tables?
A2:	No, you cannot. However, Oracle, for instance, allows you to create a cluster. You can place tables within a cluster and create cluster indexes on fields that are common to the tables. This implementation is the exception, not the rule, so be sure to study your documentation on ...