The Sample data tab shows data retrieved from the Data source using the SQL query string. The amount of data displayed can be controlled using the Rows to display field.

Rows to display

Use the Rows to display field to limit how many rows are returned as a preview of the data source. These rows of data will also be used to preview the changes on the Preview Changes tab. All available records (displayed or not) will be changed when the Go button is clicked to process the entire set of records. If you want to limit the actual rows of data returned, you can try using a 'SELECT TOP 100 * FROM <TABLENAME>' type statement (depending on your database).

Refresh button (on toolbar)

Press F5 or click the refresh button on the toolbar to refresh the preview after you have modified the transform on the Transform tab.