Retrieve duplicate values in a database table with SQL

There was an error with one of our applications, and I needed to find table rows with duplicate values. The table column was not defined to have a unique index, but we weren't anticipating duplicate values. I needed an easier way to identify duplicate values in a table with a simple SQL statement.

Consider the following "employees" table:

Assuming that we have an "employees" table with the above values, and looking to find records with duplicate "Alias". How do we retrieve them? With the following SQL statement with HAVING clause, we can easily accomplish that.

SELECT alias, count(alias) as count
FROM employee
GROUP BY alias
HAVING (count(alias) > 1)
ORDER BY alias

The above SQL statement will retrieve:

*NOTE: The HAVING clause allows SQL to use aggregate functions with a condition whereas WHERE clause does not offer that functionality. For example, the HAVING clause can be used to retrieve SUM(x) > 100 or COUNT(y) > 1.