It is highly recommended to consistently preview your results when making adjustments and additions to your Data Diagrams.  You can access the Preview Button in the Toolbar to launch the Output Preview dialog.

Figure 1:  The Output Preview dialog provides a preview of output data.

 

In the Output Preview dialog you can preview your results to ensure your diagram is returning the data you had designed it too.  The dialog will display a limited sample of your output data, but you can select Retrieve More to obtain more rows or Export XML to view the entire result.  When you are finished reviewing your sample output, select Close.