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Category: T-SQL

Ending the Number Series Generator Challenge

Published 2021-05-13 by Kevin Feasel

Itzik Ben-Gan wraps up a great series:

This is the fifth and last part in the series covering solutions to the number series generator challenge. In Part 1, Part 2, Part 3 and Part 4 I covered pure T-SQL solutions. Early on when I posted the puzzle, several people commented that the best performing solution would likely be a CLR-based one. In this article we’ll put this intuitive assumption to the test. Specifically, I’ll cover CLR-based solutions posted by Kamil Kosno and Adam Machanic.

Many thanks to Alan Burstein, Joe Obbish, Adam Machanic, Christopher Ford, Jeff Moden, Charlie, NoamGr, Kamil Kosno, Dave Mason, John Nelson #2, Ed Wagner, Michael Burbea, and Paul White for sharing your ideas and comments.

Read on for the conclusion.

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Recursive UDF Bug in SQL Server 2019

Published 2021-05-12 by Kevin Feasel

Erik Darling finds the bugs so you don’t have to:

I see people do things like this fairly often with UDFs. I don’t know why. It’s almost like they read a list of best practices and decided the opposite was better.

This is a quite simplified function, but it’s enough to show the bug behavior.

While writing this, I learned that you can’t create a recursive (self-referencing) scalar UDF with the schemabinding option. I don’t know why that is either.

I’m going to go out on a limb and say that if you run into this bug, it’s your own fault.

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Building a Better sp_help_revlogin

Published 2021-05-11 by Kevin Feasel

Eitan Blumin remembers:

Anyways, with that obvious answer out of the way, let’s talk about something more interesting, like sp_help_revlogin.

Remember sp_help_revlogin? It’s that stored procedure that Microsoft published more than 20 years ago, that never found its way into the SQL Server built-in system procedures. Microsoft still maintains that same KB page till this day (by “maintains”, I mean copy-and-pasting it from one place to another as they change their KB platforms).

Read on to understand what this is and several ways of doing the same thing better, including a new sp_help_revlogin2 that Eitan has put together.

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Fun with MERGE and Deadlocks

Published 2021-05-11 by Kevin Feasel

Daniel Hutmacher walks us through another reason to avoid using the MERGE operator:

I recently ran into a curious deadlock issue. I have a process that performs a lot of updates in a “state” table using multiple, concurrent connections. The business logic in the application guarantees that two connections won’t try to update the same item, so we shouldn’t ever run into any locking issues. And yet, we keep getting deadlocks.

What’s going on here? Hint: it has to do with isolation levels and range locks.

Read on for the problem-causing query and a few ways to resolve the problem.

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Finding Jobs Run over a Timeframe

Published 2021-05-07 by Kevin Feasel

Kenneth Fisher has a query for us:

Short post today. Simple query that will tell you every job that ran between two datetime stamps. Note: this of course assumes that the data exists. Job history is only kept for so long so depending on your settings the data may already have been deleted. Also if a job is currently running it’s first step it’s not going to show up in the history table so obviously it won’t show up in the results for this query.

Click through for the query. It uses CROSS APPLY twice, so I like it twice as much.

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Creating XML from SQL Server

Published 2021-05-07 by Kevin Feasel

Barney Lawrence shows off how to build XML from data in SQL Server:

In my experience creating XML is a less common task than reading it with one semi-exception (that being the old trick of using FOR XML PATH(”) to concatenate strings) but it can be an important one. The biggest piece of XML I’ve built was for a government mandated return, it was the only XML I built on that job but a sizeable proportion of the entire department’s work culminated in it. The switch from a set of Excel macros which took over eight hours to run to something that could rebuilt in minutes had a huge impact on our efficiency, particularly when there would be multiple rounds of validation failures against initial attempts at submission.

There are a few variants when it comes to converting queries into XML but in my opinion the only one you really need to know is the XML PATH approach, the others being either fiddly or not providing enough control.

Click through to learn more about the process. FOR XML PATH(): it’s not just for concatenating strings…

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Defining an Ad Hoc Query

Published 2021-05-05 by Kevin Feasel

Kathi Kellenberger explains what it means to be an ad hoc query:

Someone recently asked me which queries are ad hoc in SQL Server. An ad hoc query is a single query not included in a stored procedure and not parameterized or prepared. Depending on the server settings, SQL Server can parameterize some statements initially written as ad hoc queries. Ad hoc doesn’t mean dynamic.

Next on the list, a post hoc ergo propter hoc query. That’s where I explain to the DBAs that just because the server goes down every time I run a query, it doesn’t mean my queries caused this.

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Gaps and Islands in Dates

Published 2021-04-29 by Kevin Feasel

Aaron Bertrand shows off a great use for calendar tables in gap and island style queries:

In my previous article I revisited the concept of a calendar table, and explained some ways to use this data for business date calculations. This time, I wanted to explore how you can use the calendar table to simplify generating date ranges, and some query challenges this can help you simplify.

Click through for examples of the sorts of gap and island problems you can solve fairly easily with a calendar table. For an even simpler example, many BI reports want to see days even where there is no data, and a calendar table gives you that capability.

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Sequences and Filters in XPath

Published 2021-04-29 by Kevin Feasel

Barney Lawrence continues a series on XML processing in SQL Server:

This post looks at a problem that can come up in XML messages that have perhaps not been thought out as well as we’d like and where we can’t uniquely identify one instance of a repeating element. The example we’ve used so far avoids this problem so we’ll switch to a sample that illustrates this problem.

Read on for the crux of the problem, as well as solutions.

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Arithmetic Operations on DATETIME Data Types

Published 2021-04-29 by Kevin Feasel

Eitan Bluman shows off some math skills:

Mathematical addition and subtraction can be performed between two datetime data types:

SET @d2 = '1900-03-30 18:00'SELECT@d1 + @d2 -- result: 1900-04-01 10:15:15.900, @d1 - @d2 -- result: 1899-10-05 22:15:15.900, @d2 - @d1 -- result: 1900-03-29 01:44:44.100

This means that we can have basic datetime arithmetics in SQL server. We can use subtraction to find an accurate difference between two dates, and use addition to add an accurate interval to a datetime column or variable.

This is one of those things you can do, but I’m not very fond of. First of all, as Eitan points out, you can’t do these in the (in all ways superior) DATETIME2 data type. Secondly, it adds some confusion to the code, as you don’t always get what you expect.

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