There are interesting things to be learned from even the simplest queries.
For example, imagine you are asked to write a query that lists AdventureWorks product names, where the product has at least one entry in the transaction history table, but fewer than ten.
Back in 2008, Marc Friedman of the SQL Server Query Processor Team wrote a blog entry entitled “Distinct Aggregation Considered Harmful”.
Marc shows a way to work around the poor performance that often results simply from adding the keyword DISTINCT to an otherwise perfectly reasonable aggregate function in a query.
This post is an update to that work, presenting a query optimizer enhancement in SQL Server 2012 that reduces the need to perform the suggested rewrite manually.
You might have noticed that January was a quiet blogging month for me.
Part of the reason was that I was working on an article for Simple Talk, looking at how parallel query execution really works. The first part is published today at:
Understanding and Using Parallelism in SQL Server.
This introductory piece is not quite as technical as normal, but I hope there be enough interesting material there to make it worth a read.
© Paul White
email: SQLkiwi@gmail.com
twitter: @SQL_Kiwi
When you write a query to return the first few rows from a potential result set, you’ll often use the TOP clause.
To give a precise meaning to the TOP operation, it will normally be accompanied by an ORDER BY clause. Together, the TOP…ORDER BY construction can be used to precisely identify which top ‘n’ rows should be returned.