How to Combine DISTINCT and ORDER BY for Unique, Ordered Results in SQL?

How to Use DISTINCT and ORDER BY in the Same SELECT Statement

Executed queries often return duplicate data, and ordering that data using ORDER BY alone may not be enough to remove these duplicates. By combining DISTINCT and ORDER BY in a single SELECT statement, however, developers can efficiently eliminate duplicates while maintaining the desired order.

To address this, the ORDER BY clause must include the columns used in the SELECT clause. When DISTINCT is used, it groups the results based on the specified columns, effectively removing duplicates. However, if the ORDER BY clause refers to columns not included in the DISTINCT, the ordering will incorrectly group all rows with the same value in the SELECT columns, leading to duplicate results.

Solution:
To resolve this issue, the query should use an aggregate function, such as MAX(), to sort on the desired column (CreationDate in this case). Additionally, a GROUP BY clause is added to group the results by the DISTINCT columns (Category). This combination ensures that duplicates are removed while preserving the order of the results. The modified query would be as follows:

SELECT DISTINCT Category, MAX(CreationDate)
FROM MonitoringJob
GROUP BY Category
ORDER BY MAX(CreationDate) DESC, Category

This revised query will output the desired results, eliminating duplicates from Category and ordering them by CreationDate in descending order, followed by Category.

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