Add a working SQL query. Do no use the moodle database prefix $CFG->prefix instead use "prefix_" without quotes

Example: SELECT * FROM prefix_course

You can find a lot of SQL Reports here: ad-hoc contributed reports

Since this block supports Tim Hunt's CustomSQL Queries Reports, you can use any query.

Remember to add a "Time filter" if you are going to use reports with time tokens.

For using filters see: Creating a SQL Report Tutorial