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