24 : Understanding Databases
Linking indexed tables When you are linking direct-access database tables, you link from a primary table to a field in the lookup table. The link field in the primary table can be indexed, but does not have to be. The link field in the lookup table does not have to be indexed, either, unless you aren’t using a native driver for your connection. In addition, the fields used to link two tables must have the same data type. For example, you can link a string field in one table to a string field in another table, or a numeric field in one table to a numeric field in another table, but you cannot link a numeric field in one table to a string field in another table. Note: • Some DBMS applications allow you to convert the field value to another data type in the index. For instance, the field in the table can be numeric, while the index converts the field value to a string. However, if you choose to use that field to link to another table, you must link to a field of the original data type. You cannot link a string value to a numeric field that has been converted to a string in the index. • If you are linking tables from two different ODBC data sources, MS SQL Server and Oracle, for example, you can only use string fields to link the tables. String fields are stored in databases in the same manner, regardless of the data source. Other types of values, however, may not be stored the same way in different data sources, so you cannot link different data sources in Crystal Reports using anything other than string values. • When linking direct-access database tables using native drivers (non-SQL), the only join type available is Left Outer join. Changing the index used in linking When using the Smart Linking feature to link tables using a field that is a component of multiple indexes (two or more), Crystal Reports selects one of the indexes for the link. That index may or may not be the one you want to use. Delete the link made by the Smart Linking feature and manually link the tables. See for Linking multiple tables more information. Note: Not all DBMS applications support indexed tables. Verify that your database uses indexes before trying to select an index for linking. Refer to your DBMS documentation to find out if your DBMS can use indexes, and to learn how to create them.
Link processing order When there is more than one link, Crystal Reports needs to know in what order it should process the links. By default, the processing order matches the order in which the links appear on the Links tab. Use the arrow buttons in the Links Order dialog box to change the default order.
Crystal Reports User’s Guide
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