HP Intelligent Management Center Standard Software Platform - Manual (Page 665)

since information in the index is organized according to the customer names,
the database engine does not need to continue searching through the index
or the table as soon as it finds an index entry that does not match the
requested customer.
The advantage of this highly organized search through a database table
according to an index is speed. Using indexes speeds up data retrieval and
report generation, important factors when reporting on large database files.
Linking tables
You link tables so records from one table will match related records from
another. For example, if you activate an Orders table and a Customers table,
you link the tables so that each order (from the Orders table) can be matched
up with the customer (from the Customer table) that made the order.
When you link, you are using a field that is common to both tables. Crystal
Reports uses the link to match up records from one table with those from
the other. In this example, the link assures that the data in each row of the
report refers to the same order.
Link from and link to
When you link two tables, you link from one table to another table. The from
table is used as a primary table, while the to table acts as a lookup table
where records are looked up by the primary table. In a simple link, the Report
Designer examines the first record in the primary table and finds all matching
records in the lookup table. Once all matches have been found in the lookup
table for the first record in the primary table, all matches in the lookup table
for the next record in the primary table are found.
Link relationships
When you link records from one table to another table, the records will
typically fall under one of two relationship types: one-to-one or one-to-many.
Crystal Reports 2008 SP3 User's Guide 665
25 Understanding Databases
Linking tables