I admit HAVING is a basic SQL-element that everyone should know, but one good example is all the poor TS needs.
Instead of feeling superior it would be better to just help the guy.
SELECT COL1 from tab1 GROUP BY COL1 HAVING COUNT(*) > 1
would give you all the COL1 with multiple rows in the table.
It won't give you all the rows, just all the distinct COL1 values .
If you actually need all the corresponding rows, you would need to join this result with the original table.