I needed a query to find records where duplicate name values exist. I know how to group by and count, but the table has millions of records. I don’t want the 99% of the data where no duplication occurs. By using “having” in conjunction with “group by”, I am able to restrict the output to the groups that match my criterion.

select display_name, count(display_name) from circuit
group by display_name
having count(display_name) > 1;

My result set is the display name & occurrence count for that display name without all of the ‘good’ records where there’s a unique display name. (Yes, I know uniqueness could be enforced. The real scenario isn’t this straight-forward. There are times where the display name should be the same and I’ve got additional filters that drop out those cases).