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Category: T-SQL

T-SQL Query Transformations and Performance

Published 2023-07-12 by Kevin Feasel

Erik Darling isn’t satisfied with “good enough”:

Query tuning is hard work, especially when queries run for a long time and you have to test various changes trying to get it down to a reasonable runtime.

There are, of course, things you pick up intuitively over time, and figuring out where problems in actual execution plans has gotten easier with operator runtimes.

Beyond basic query tuning intuition comes the really creative stuff. The stuff you’re amazed anyone ever thought of.

Click through for two really interesting examples.

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Using Subqueries in a SELECT Statement

Published 2023-06-27 by Kevin Feasel

Greg Larsen builds a subquery:

Did you know you can include a SELECT statement within another SELECT statement? When a SELECT statement is embedded within another statement it is known as a subquery. There are two types of subqueries: basic subquery and correlated subquery.

In this article I will be discussing both types of subqueries and will be providing examples of how to use a subquery in different places within in a SELECT statement.

Greg has a good write-up on the topic of subqueries and does well to separate correlated from non-correlated subqueries.. And if you want to learn more about those, as well as common table expressions, I put out a video on the topic just last week.

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Finding the Actual Error Line in sp_executesql

Published 2023-06-23 by Kevin Feasel

Thom Andrews solves a problem:

Notice that the error line states line 2 not line 8, which is the line the sys.sp_executesql was called on. Knowing the line the error occured on within the dynamic batch is certainly important, but if you’re working with multiple dynamic batches you have no way of knowing which dynamic batch might have produced the error; was it the one executed on line 100? Line 200? Line 350?

Solving this problem wasn’t wasn’t exactly simple, and it came with a couple of caveats.

Click through for the approach. As Thom mentions, it isn’t perfect, but it is reasonable and interesting.

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The Value of QUOTENAME

Published 2023-05-31 by Kevin Feasel

Quoth Chad Callihan, “Occasionally more”:

QUOTENAME can be used to make sure database objects are valid in your query. Most of the time, objects like table names only contain valid characters, so there’s nothing to worry about. But nobody’s perfect. Let’s look at an example of what can happen when somebody creates a table with a forward slash in the name and see how QUOTENAME can be used to query against it.

QUOTENAME is also a good way of preventing SQL injection, though I still prefer appropriate use of exec sp_executesql in any case in which it’s possible to use.

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Filling Gaps in T-SQL

Published 2023-05-30 by Kevin Feasel

Itzik Ben-Gan has a new challenge:

The task involves developing a stored procedure called GetBalances that accepts a parameter called @accountid representing an account ID. The stored procedure should return a result set with all existing dates and balances for the input account, but also gap-filled with the dates of the missing workdays between the existing minimum and maximum dates for the account, along with the last known balance up to that point. The result should be ordered by the date.

My first thought was last observation carried forward, which is now available in SQL Server 2022 (Itzik’s solution 2). I kind of thought of solution 3, though did not think through the mechanics of how it’d work and so I get no credit there.

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Trying NTILE

Published 2023-05-17 by Kevin Feasel

Chad Callihan looks at the fourth ranking window function:

Have you ever used the NTILE function? Or have you even heard of the NTILE function? It seems to be one of the lesser known, lesser used window functions in SQL Server. I’ve never come across it in the wild but maybe there are those that use it all the time. Either way, let’s have a look at what it does and how it can be used.

Click through for a demo. I definitely use it a lot less than ROW_NUMBER(), RANK(), and DENSE_RANK(), but I have used it to some good effect in the past, mostly in cases where I’ve wanted to focus on the top X% of data for an analysis.

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Choosing from a List of Values

Published 2023-05-10 by Kevin Feasel

Greg Dodd doesn’t need no steenkin’ tables:

Sometimes you have a list of values or parameters that you’d like to run a select statement over the top of to get a result. There’s a few ways of doing this, usually I see people create a Temporary Table and insert the data. But is there an easier way?

Yes, we can select from VALUES:

The result of this is called a virtual table, and as far as other parts of the query are concerned, it’s just another table.

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