Category: T-SQL

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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Double-Join Syntax in T-SQL

Published 2021-04-27 by Kevin Feasel

Greg Dodd unleashes a monster:

We now have all of the data, but the question is, how do we stop the Person’s name coming back if the HireDate is after 1 Jan 2000?
We can’t simply put it in a where clause, because then the Course Title won’t come back. We could put it as part of the join condition to Instructor, but that will only limit the hire date coming back. We could use that to build a case statement around the FirstName and LastName fields, but what other options are there?
What we want to do is limit the join to Person to only return if Instructor returns, and then we could put the join condition on Instructor.

I’ll admit that whenever I see this syntax, I tend to remove it and replace with a subquery or APPLY operation. I don’t like multi-joins at all for the reasons Greg mentions at the end: it’s uncommon and difficult for a T-SQL developer to parse mentally.

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Writing Check Constraints in SQL Sever Data Tools

Published 2021-04-27 by Kevin Feasel

Chris Johnson has a how-to guide:

That all looks ok, and everything published fine, and kept on publishing fine until I happened to check the generated script for some of the publishes (I was in the early stages of development at this point so deployments were just being done through Visual Studio direct to my machine).
When I did I noticed that every deployment I was getting code like this:
That’s dropping all my constraints, recreating them using WITH NOCHECK, and then using WITH CHECK to check the existing data. Obviously all the data will pass those checks, but that’s going to add some time to my deployments, and as the size of the data and the number of checks both increase, these checks will take more and more time.

Read on to understand what’s happening. I’d call this a fairly silly limitation on the part of SSDT.

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OFFSET and FETCH in SQL Server

Published 2021-04-22 by Kevin Feasel

Steve Jones explains how the OFFSET-FETCH process works:

The other day I saw an article on the OFFSET clause in a SELECT. I had seen this come out and looked at it briefly in SQL Server 2012, but hadn’t done much with it.
NOTE: if you use this, be sure you read about potential performance problems and solutions.

Read on for more info. This is something I really wanted to work better than it does, as paging is really awkward in SQL Server. OFFSET-FETCH is great syntactically, but doesn’t do much for performance. The best solution I’ve seen is to take the results of a paging operation and store them in a dedicated paging table, allowing the user to query by page in that small table quickly. But at that point, OFFSET-FETCH isn’t really much less complicated than TOP(NumberOfRows) WHERE NumberOfRows >= NumberOfRows * (NumberOfPages – 1) ORDER BY NumberOfRows.

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QUOTENAME and Dynamic SQL

Published 2021-04-20 by Kevin Feasel

Chris Johnson looks at one way to protect dynamic SQL statements:

I don’t have the usual aversion to dynamic SQL that you find a lot of developers have. I use it quite regularly as part of my ETL processes, usually to define how data will move from one stage to another. So, for instance, I might have a single Validate procedure that takes a number of parameters, including an import table name, and moves data from that import table to a valid table according to my validation rules. Or I might have a single SCDMerge procedure that takes a transformation view and merges that into a slowly changing dimension (although not using the actual MERGE function for various reasons). These procedures allow me to have confidence that data will always move from one stage to another in the same way, and saves me from writing essentially the same statement 50 times, and having to update it 50 times when I need to change the way we do something, and inevitably missing something and introducing some error.
This always feels like a pretty safe use of dynamic SQL to me, because it avoids some of the more common objections people like to raise to it:

Click through to see how QUOTENAME() can help sanitize user inputs. I personally prefer the route of using sp_executesql but QUOTENAME() can also do the trick.

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Alternatives to GREATEST() and LEAST()

Published 2021-04-19 by Kevin Feasel

Mike Scalise has an alternative to using the GREATEST() and LEAST() functions in SQL Server:

As of 4/14/21, Microsoft has officially announced that the GREATEST and LEAST functions are in Azure SQL Database and SQL Managed Instance. Unofficially, it seems they had silently included these functions in at least (no pun intended) SQL Managed Instance several months prior. In any case, here we are today with official Microsoft documentation on GREATEST and LEAST. This is all great news. What’s also great is that, in their statement, Microsoft stated they would be including these two functions in the next version of SQL Server.
But what about all of us on SQL Server 2019 and prior? Fortunately, there’s a way to mimic these two functions in your queries using a correlated subquery in the SELECT clause.

Click through for examples. This is a bit different from getting the largest or smallest value in a window, which you can do with MIN(val) OVER () or MAX(val) OVER (). But I’m looking forward to seeing GREATEST() and LEAST() in the box product.

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