Sometimes you need to find and count duplicate data rows in SQL. For example, in my use case I needed to find records in a table where there was more than one usage of the same email address. This would help me figure out how widespread and severe the duplicate issue was; the table in question should not have had duplicate rows based on that column in the first place! (A missing UNIQUE index was the culprit).

SELECT email, COUNT(*)
FROM user_accounts
GROUP BY email

The HAVING clause is the important part of this query. To find duplicates, we need to check if any of the groups have a record count > 1. You can put other conditions for the groups in the HAVING clause as well if required, e.g. COUNT(*) > 1 AND account_status = 1.

The result of this query can then be used for a sub query/WHERE clause. The result looks like:

email              | count
--------------------------------  | 2 | 3