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Category: T-SQL

The Concatenation Operator

Published 2021-01-19 by Kevin Feasel

Hugo Kornelis explains what the Concatenation operator does:

The Concatenation operator reads and returns all rows from all its inputs, in order, and without modification.

This operator is most commonly used to execute queries that use UNION or UNION ALL. In the former case, other operators are required to remove the duplicates, because Concatenation doesn’t provide that functionality. You may also find the Concatenation operator in queries on partitioned views.

Read on to see the algorithm and lots of details about the operator.

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Build Your Own Number Generator

Published 2021-01-14 by Kevin Feasel

Itzik Ben-Gan has part one of solutions to a challenge:

Last month I posted a challenge to create an efficient number series generator. The responses were overwhelming. There were many brilliant ideas and suggestions, with lots of applications well beyond this particular challenge. It made me realize how great it is to be part of a community, and that amazing things can be achieved when a group of smart people joins forces. Thanks Alan Burstein, Joe Obbish, Adam Machanic, Christopher Ford, Jeff Moden, Charlie, NoamGr, Kamil Kosno, Dave Mason and John Number2 for sharing your ideas and comments.

Initially I thought of writing just one article to summarize the ideas people submitted, but there were too many. So I’ll split the coverage to several articles. This month I’ll focus primarily on Charlie’s and Alan Burstein’s suggested improvements to the two original solutions that I posted last month in the form of the inline TVFs called dbo.GetNumsItzikBatch and dbo.GetNumsItzik. I’ll name the improved versions dbo.GetNumsAlanCharlieItzikBatch and dbo.GetNumsAlanCharlieItzik, respectively.

One of the best parts about this happening in public is that there is a real benefit to having multiple people look over a problem, and then waves of people refining those solutions over time.

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Archival on Delete in SQL Server

Published 2021-01-12 by Kevin Feasel

Erik Darling shows off a pattern:

Well, friends, I have good news for you. This is an easy one to implement.

Let’s say that in Stack Overflow land, when a user deletes their account we also delete all their votes. That’s not how it works, but it’s how I’m going to show you how to condense what can normally be a difficult process to isolate into a single operation.

The one gripe I have with this post is that my annoyingly loud keyboard is buckling spring, not Cherry MX Blue, thank-you-very-much.

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String Modification in T-SQL

Published 2021-01-08 by Kevin Feasel

Steve Jones answers a question:

Recently I ran across a question posted by a beginner on the Internet and thought this would be a good, basic topic to cover. The question was: how can I replace a value in a comma separated string in a table?

This post covers the basics of this task.

Incidentally, this is where I say hey, that sounds like a failure in normalization. If you need to care about individual values in a collection, your value is not atomic. But that’s a bit of a tangent.

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Performance Impact of Foreign Keys with Non-Default ON UPDATE or ON DELETE

Published 2021-01-06 by Kevin Feasel

Hugo Kornelis continues a dive into foreign keys:

Welcome to part fifteen of the plansplaining series. In the three previous parts I looked at the operators and properties in an execution plan that check a modification doesn’t violate foreign key constraints. That part is done. But I’m not done with foreign keys yet.

We normally expect foreign keys to throw an error on violations. But that’s actually only the default option: they can also be set to be self-correcting. This is done using the ON UPDATE and ON DELETE clauses, which provide the user with several choices on how to handle child data that would become orphaned, and hence violate the constraint, as a result of a change in the parent table.

Read on to see how these operate in SQL Server.

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Correlated Subqueries which Don’t

Published 2020-12-21 by Kevin Feasel

Daniel Hutmacher gives us an eye test:

The developer wrote this pretty little query to show us which accounts are up for review (which in our case means they have a “30” flag).

SELECT account, balance, 'For review' AS [status]
FROM #accounts WHERE account IN (SELECT account FROM #accountFlags WHERE flag=30) ORDER BY account;

Did you spot it?

I did, but in fairness, I’ve been burned enough times by this that I check for it.

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T-SQL Additions to Serverless SQL Pools

Published 2020-12-21 by Kevin Feasel

Jovan Popvic lays out some of the T-SQL syntax added to serverless SQL pools in Azure Synapse Analytics:

Serverless Synapse SQL pools in Azure Synapse Analytics have a new set of features that will enable you to analyze your Azure data more efficiently. The new Transact-SQL (T-SQL) language features that you can use in serverless SQL pools are STRING_AGG, OFFSET/FETCH, PIVOT/UNPIVOT, SESSION_CONTEXT, and CONTEXT_INFO.

Old T-SQL hands will likely know what all of this does, but click through if something looks unfamiliar. All of this is available in SQL Server 2017 and later (and everything but STRING_AGG() is available going back to 2008).

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A Use Case for Recursive CTEs

Published 2020-12-17 by Kevin Feasel

Jeffin Mathew takes us through a use case for recursive common table expressions:

An individual is working in HR and wants to find out which individual is managing who. This may be for several reasons such as, they need to ask the managers on the progress of their staff and if their appraisal is coming up or is due.

Another scenario may be that the company is enrolling more staff and wants to find out the capacity of the current staff or find individuals who have not yet got anyone to manage to give them the opportunity to do so.

Click through for the solution. Often times, we see recursive CTEs show up in hierarchical queries like this. When the number of records is small, they work really well. The issue comes with scale; that’s when a different table design becomes important.

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