The report may contain data sources that are added using the Data Wizard in Designer (via "Data|Add Data Source..." menu).
Sometimes it is needed to pass custom SQL to that data source from your application. To do this, use the following code:
using FastReport.Data;
...
report1.Load(...);
// do it after loading the report, before running it
// find the table by its alias
TableDataSource table = report1.GetDataSource("MyTable") as TableDataSource;
table.SelectCommand = "new SQL text";
report1.Show();Passing Own Connection String | Top Page | Reference to a Report Object