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Category: T-SQL

Using GREATEST and LEAST in Azure SQL DB

Published 2021-12-10 by Kevin Feasel

Aaron Bertrand preps us for SQL Server 2022:

In an earlier tip, “Find MAX value from multiple columns in a SQL Server table,” Sergey Gigoyan showed us how to simulate GREATEST() and LEAST() functions, which are available in multiple database platforms but were – at least at the time – missing from Transact-SQL. These functions are now available in Azure SQL Database and Azure SQL Managed Instance, and will be coming in SQL Server 2022, so I thought it was a good time to revisit Sergey’s methods and compare.

Read on to see how the workaround compares.

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Batch Mode and Window Functions

Published 2021-12-07 by Kevin Feasel

I wind down a series on window functions:

SQL Server typically operates in row mode, which means that an operator processes one row at a time. This sounds inefficient, but tends to work out pretty well in practice. However, something which may work out even better is to process more than one row at a time, especially when the number of rows gets to be fairly large. Enter batch mode.

Batch mode was introduced in SQL Server 2012 alongside non-clustered columnstore indexes. It became interesting in SQL Server 2016 and very interesting in SQL Server 2019. That’s because 2016 introduced writable clustered columnstore indexes and 2019 gives us batch mode outside of columnstore indexes.

There are some nice potential performance gains for queries involving window functions.

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Indexing and Window Functions

Published 2021-12-06 by Kevin Feasel

I continue a series on window functions in SQL Server:

If you’ve been around the block with window functions, you’ve probably heard of the POC indexing strategy: Partition by, Order by, Covering. In other words, with a query, focus on the columns in the PARTITION BY clause (in order!), then the ORDER BY clause (again, in order!), and finally other columns in the SELECT clause to make the index covering (not in order! though it doesn’t hurt!).

But do read on to understand why this is not sufficient.

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Building 2048 in T-SQL

Published 2021-11-30 by Kevin Feasel

Tomaz Kastrun gives you a way to slack off at work while everybody else thinks you’re working on a really important SQL problem:

What is 2048 game? It is a classical puzzle game, that is easy and fun to play. The objective of the game is to move the numbers (tiles in the matrix/board) in a way to combine them to create a tile with the number 2048.

Click through to see how to use it and check out the scripts on Tomaz’s GitHub repo. This definitely merits the Wacky Ideas category.

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Use Cases for Window Functions

Published 2021-11-30 by Kevin Feasel

I continue a series on window functions in SQL Server:

To this point, we’ve looked at five classes of window function in SQL Server. I’ve given you a couple of solid use cases, but for the most part, we’ve focused on what the classes of window functions are. Now we’re going to talk about why you want to use them.

As is the norm for me, it takes about seven posts before I explain why you might want to do something.

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CONVERT and Binary Styles

Published 2021-11-30 by Kevin Feasel

Abayomi Obawomiye has style:

I recently had a requirement to load some data from a source table to another destination table. The destination columns were exactly the same as the source columns with the same data types and length. The only difference was that some columns on the destination table must be encrypted. The task was to use the SHA2_512 encryption algorithm to encrypt the “sensitive” data. I will talk more about the encryption algorithm in another post.

To achieve this, I needed to use the HASHBYTES function in SQL Server. The challenge was that this function used with the SHA2_512 encryption algorithm will return a fixed character length of 64 characters which will be longer than the character length on my destination table. As a result, SQL Server will throw a truncation error. I will demonstrate this below.

One really important point: SHA is not encryption; it’s a hash (which is why the function is HASHBYTES() instead of something like EncryptByKey() as column-level security uses). Hashes are intended to be a one-way trip, whereas encryption implies an ability to decrypt if you have the relevant key details. Here, the use looks to be obfuscating the text of sensitive data fields, perhaps for loading in a dev/test environment, and so the actions themselves are quite reasonable.

By the way, the styles Abayomi talks about are all listed in this Docs page. Turns out that if you’re using a money datatype, you can use CONVERT() to display the end result with commas.

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Ordered Set Functions in SQL Server

Published 2021-11-29 by Kevin Feasel

I continue a series on window functions in SQL Server:

As of SQL Server 2019, there is only one ordered set function: STRING_AGG(). I like STRING_AGG() a lot, especially because it means my days of needing to explain the STUFF() + FOR XML PATH trick to concatenate values together in SQL Server are numbered.

STRING_AGG() is interesting in that we categorize it as a window function and yet it violates my first rule of window functions: there isn’t an OVER() clause. Instead, it accepts but does not require a WITHIN GROUP() clause. Let’s see it in action.

Click through for a look at that, as well as a little hint that maybe we’ve seen ordered set functions before in a different guise.

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FAST_FORWARD and Cursors

Published 2021-11-29 by Kevin Feasel

Joe Obbish skips past the commercials:

If you’re like me, you started your database journey by defining cursors with the default options. This went on until a senior developer or DBA kindly pointed out that you can get better performance by using the FAST_FORWARD option. Or maybe you were a real go-getter and found Aaron Bertrand’s performance benchmarking blog post on different cursor options. I admit that for many years I didn’t care to know why FAST_FORWARD sometimes made my queries faster. It had “FAST” in the name and that was good enough for me.

Recently I saw a production issue where using the right cursor options led to a 1000X performance improvement. I decided that ten years of ignorance was enough and finally did some research on different cursor options. This post contains a reproduction and discussion of the production issue.

I thought everybody knew how this works: the database streams the data tape from the supply pully to the play shaft by using a sprocket to rotate the gear in the cassette at a fixed speed. The FAST_FORWARD cursor option engages the fast forward idler in the VCR database and causes rotation to occur more rapidly than normal.

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Statistical Window Functions in SQL Server

Published 2021-11-26 by Kevin Feasel

I continue a series on window functions in SQL Server:

CUME_DIST() doesn’t show 0 for the smallest record. The reason for this is in the definition: CUME_DIST() tells us how far along we are in describing the entire set—that is, what percentage of values have we covered so far. This percentage is always greater than 0. By contrast, PERCENT_RANK() forces the lowest value to be 0 and the highest value to be 1.

Another thing to note is ties. There are 117 values for customer 1 in my dataset. Rows 5 and 6 both have a percent rank of 0.0344, which is approximately rank 4 (remembering that we start from 0, not 1). Both rows 5 and 6 have the same rank of 4, and then we move up to a rank of 6. Meanwhile, for cumulative distribution, we see that rows 5 and 6 have a cumulative distribution of 6/117 = 0.5128. In other words, PERCENT_RANK() ties get the lowest possible value, whereas CUME_DIST() ties get the highest possible value.

Click through for much more detail, including examples galore.

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