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Category: T-SQL

Upsert Patterns and Duplicate Keys in T-SQL

Published 2025-06-30 by Kevin Feasel

Ajay Dwivedi runs into an error:

I work with developers quite a lot. There are scenarios where data is received from various sources in an application, and asynchronously pushed to database in multiple sessions/connections.

There are situations where concurrency and transaction speed is high enough to cause below UPSERT code blocks to fail with error message like Cannot insert duplicate key row in object dbo.person with unique index ‘pk_person’

Click through for one way to do things. I will note that Ajay has concerns about the MERGE operator, but Hugo Kornelis took a deep dive into all of the known problems in MERGE and found that most of them were fixed. Hugo’s post does make it clear when using MERGE is a bad idea, as there are still some situations in which it won’t work effectively, but for something like this, it would be fine.

I’ll say that I’m generally not a fan of app locking. There are specific circumstances in which it’s the best answer, but those are rare. Here, I’d rather just use a ROWLOCK table hint or change the serialization level.

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The Costs of Implicit Conversion

Published 2025-06-27 by Kevin Feasel

Andy Brownsword changes things around:

Implicit conversion happens in SQL Server when the engine detects a mismatch in data types and automatically converts from one type to another. This can be helpful as it makes different types interchangeable and is generally transparent to the client, but it can come with issues.

Usually the downside from implicit conversion is seen through performance impact. But we’ve got something a little different today – an inconsistent result set.

Read on for Andy’s example.

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Inline Scalar UDFs in Microsoft Fabric Warehouse

Published 2025-06-24 by Kevin Feasel

Srdjan Martin has an announcement:

SQL native Scalar user-defined functions (UDFs) in Microsoft Fabric Warehouse and SQL analytics endpoint are now in preview.

A scalar UDF is a custom code implemented in T-SQL that accepts parameters, performs an action such as complex calculation, and returns a result of that action as a single value.

I wonder if the same advice for SQL Server will apply to these: don’t use them because they are performance sinks. Knowing that these are scalar functions, I’d be inclined to do some considerable performance testing before rolling them out.

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Building a Better Trigger

Published 2025-06-19 by Kevin Feasel

Steve Jones is done with mousetraps:

This seemed to work, but did it really?

The Problem

Let’s illustrate the big problem with this change. I’ll run this code:

Steve hit on one of the problems in trigger design, but there’s another problem that we often see. Steve’s code doesn’t have this problem, but quite often, people assume that rows update one at a time. But if you write a T-SQL UPDATE statement that affects five rows, the trigger fires once for all five. Steve correctly uses the inserted and deleted pseudo-tables to handle all changes, rather than just assuming a single row.

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Reshaping Data with the APPLY Operator

Published 2025-06-11 by Kevin Feasel

I have a new video:

In this video, I show how we can use the APPLY operator to reshape datasets, allowing us to unpivot tables and also calculate the greatest and least values for a row.

If you look closely at the scripts, you’ll see 08 and 10. In the source control repo, I also have a script 09 that covers splitting strings. Using APPLY to split strings has always been a bit of a niche case, but prior to SQL Server 2016’s introduction of STRING_SPLIT() and SQL Server 2022’s improvement of the function, I could make the case that it sometimes made sense to know how to split strings via APPLY. Today, not so much, which is why I tossed that demo from the video.

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Simplifying Calculations with the APPLY Operator

Published 2025-06-04 by Kevin Feasel

I have a new video:

In this video, I show how we can use the APPLY operator to remove redundancy in the SELECT clause and simplify complex calculations, all with zero performance impact.

This is, as I’ve said in the past, my favorite use case for the APPLY operator. As I’ve become older and (even) more crochety, I’ve sided more and more with “easy to read” versus “runs faster” for code. And when you get “easy to read” with no impact on “runs faster,” I’m all in.

The accounting scenario I show may be a fairly extreme case, but I’d bet that queries similar to this abound in almost any company. A big part of why complex reporting queries are so complex comes from repetition of calculations.

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Retrieving Child Records with APPLY

Published 2025-05-28 by Kevin Feasel

I have a new video:

In this video, I show how we can use the APPLY operator to find records associated with a “parent” table. We look at getting the latest record, as well as any arbitrary number of records. We also spend time digging into under what circumstances the APPLY operator out-performs other common options.

During the outro segment, I dig into under what circumstances APPLY is the best tool for the job, so stick around for the end.

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Combining DISTINCT and UNION

Published 2025-05-27 by Kevin Feasel

Louis Davidson gives it the college try:

When I was perusing my LinkedIn feed the other day, I came across this thread about using SELECT *. In one of the replies, Aaron Cutshall noted that: “Another real performance killer is SELECT DISTINCT especially when combined with UNION. I have a whole list of commonly used hidden performance killers!”

To which started my brain thinking… What does happen when you use these together? And when you use UNION on a set with non-distinct rows, what happens. So for the next few hours I started writing.

Read on for Louis’s findings.

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