Adding data fields for an expression
It is common to use a data field in an expression for a computed column. As mentioned earlier, a computed column only has access to data fields that are used in the report design. Sometimes, however, you need to write an expression that uses a field provided by the data set, but you do not want to display the field values in the report.
For example, if you insert a CustomerName field and a Phone field in a table, then you insert a computed column, the computed column only has access to the CustomerName and Phone fields. You cannot create an expression that combines, for example, values from the AddressLine1, City, State, and PostalCode fields. To create such a computed column, you must first add the data fields to the table, without actually inserting the fields in the table.

Additional Links:

Copyright Actuate Corporation 2013 BIRT iHub 2