How Can I Find Duplicate Email Addresses in a Database Table Without Using DISTINCT?

Identifying Duplicates in Column Values Without DISTINCT

In a database, it may be necessary to identify rows where a specific column's value is repeated. Unlike the DISTINCT keyword, which only retrieves unique values, the task at hand requires the selection of all rows where the column value lacks distinctiveness.

One approach to address this requirement is to use a subquery within the WHERE clause. The following query demonstrates this technique:

SELECT [EmailAddress], [CustomerName]
FROM [Customers]
WHERE [EmailAddress] IN
  (SELECT [EmailAddress]
   FROM [Customers]
   GROUP BY [EmailAddress]
   HAVING COUNT(*) > 1)

This query retrieves all rows from the 'Customers' table where the 'EmailAddress' value exists in a group with more than one instance. By comparing the 'EmailAddress' values in the outer query to those in the subquery, it effectively filters out unique values.

For the sample data provided, the above query would produce the desired result:

CustomerName	EmailAddress
Aaron	[email protected]
Christy	[email protected]
John	[email protected]

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