Up until this point, only reports based on a single table, view, stored procedure, or SQL Command have been discussed. However, it is quite common to have several disparate database objects in the same report. Crystal Reports treats all types of database objects as peers, which means that a single report can contain multiple tables, views, stored procedures, and SQL Commands. Because all database objects are treated as peers, the term table will be used from now on to describe any of these database objects.
Because of Crystal Reports' inherent basis on relational data, any time multiple tables are used, they must be linked together so that the sum of all database objects is a single set of relational records. The good news is that most of the time, Crystal Reports takes care of this automatically, and the report developer need not worry about linking.
To see this in action, create a connection to the Xtreme Sample Database and add both the Customer and Orders tables to the report. When clicking Next in the Report Wizard, the linking between those tables is displayed as shown in Figure 1.11. Each table is represented by a window. In addition to the name, each field in the table is listed inside the window, and those fields that are defined as indexed fields in the database are marked with colored arrows. Any links defined between tables are represented as arrows connecting the key fields from two tables. Based on general database theory, linking to a field that is indexed generally results in a better performing query, and indexing is highlighted in this dialog through the color-coded icons displayed beside the field names.
By default, Crystal Reports creates links based on name. In this case, both tables have fields with a name of Customer ID, so a link is already created. To accept this link, simply click Next to move to the next step in the Report Wizard. If there were not a common field name, selecting the By Key option and clicking Auto-Link would attempt to create a link based on the fields defined in the database as keys. If neither of these methods of automatic linking work, the link must be manually created. This is very simple to do: Simply drag the field to link from one table and drop it onto the field from a second table.
After links are created, you can configure them by clicking on the link arrow connecting two tables (it turns blue when selected), and then clicking the Link Options button. Links have two options: join type and link operator. These settings determine how Crystal Reports matches records from both tables. The default join type is an inner join, which means that only records with a matching key in both tables are included. The default link type is equal. For most cases, these two settings do not need to be modified.