In SSRS, when the dataset of a subreport contains no data, the subreport won't show in your main report. You can use the NoRows property of in the main report to show a message, but maybe you want the controls of your subreport to be visible (the table headers,...)
The following hack can achieve this:
Add a new DataSet to your subreport
Choose a DataSource (even though we won't use it) and set the following as query:
SELECT ' ' AS DUMMY
Or for Oracle (PL/SQL):
SELECT ' ' AS DUMMY FROM DUAL
Now add a Textbox to your subreport and set its value to:
Leave the textbox as visible or it won't work (at least, it didn't for me). That's why I took an empty string and not a number (i.e.
Now, even when your main DataSet doesn't have data, your subreport will have data and so your subreport will always be visible.