How to use DISTINCT and ORDER BY in same SELECT statement?

The problem is that the columns used in the ORDER BY aren't specified in the DISTINCT. To do this, you need to use an aggregate function to sort on, and use a GROUP BY to make the DISTINCT work.

Try something like this:

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

Extended sort key columns

The reason why what you want to do doesn't work is because of the logical order of operations in SQL, which, for your first query, is (simplified):

  • FROM MonitoringJob
  • SELECT Category, CreationDate i.e. add a so called extended sort key column
  • ORDER BY CreationDate DESC
  • SELECT Category i.e. remove the extended sort key column again from the result.

So, thanks to the SQL standard extended sort key column feature, it is totally possible to order by something that is not in the SELECT clause, because it is being temporarily added to it behind the scenes.

So, why doesn't this work with DISTINCT?

If we add the DISTINCT operation, it would be added between SELECT and ORDER BY:

  • FROM MonitoringJob
  • SELECT Category, CreationDate
  • DISTINCT
  • ORDER BY CreationDate DESC
  • SELECT Category

But now, with the extended sort key column CreationDate, the semantics of the DISTINCT operation has been changed, so the result will no longer be the same. This is not what we want, so both the SQL standard, and all reasonable databases forbid this usage.

Workarounds

It can be emulated with standard syntax as follows

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

Or, just simply (in this case), as shown also by Prutswonder

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

I have blogged about SQL DISTINCT and ORDER BY more in detail here.

Tags:

Sql

Distinct

Sql Order By

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