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Category: Syntax

The Concatenation Operator

Published 2021-01-19 by Kevin Feasel

Hugo Kornelis explains what the Concatenation operator does:

The Concatenation operator reads and returns all rows from all its inputs, in order, and without modification.

This operator is most commonly used to execute queries that use UNION or UNION ALL. In the former case, other operators are required to remove the duplicates, because Concatenation doesn’t provide that functionality. You may also find the Concatenation operator in queries on partitioned views.

Read on to see the algorithm and lots of details about the operator.

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Using sp_prepare with Plan Guides

Published 2021-01-13 by Kevin Feasel

Aaron Bertrand tries something different:

There are features many of us shy away from, like cursors, triggers, and dynamic SQL. There is no question they each have their use cases, but when we see a trigger with a cursor inside dynamic SQL, it can make us cringe (triple whammy).

Plan guides and sp_prepare are in a similar boat: if you saw me using one of them, you’d raise an eyebrow; if you saw me using them together, you’d probably check my temperature. But, as with cursors, triggers, and dynamic SQL, they have their use cases. And I recently came across a scenario where using them together was beneficial.

Read on to see the method to this madness.

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String Modification in T-SQL

Published 2021-01-08 by Kevin Feasel

Steve Jones answers a question:

Recently I ran across a question posted by a beginner on the Internet and thought this would be a good, basic topic to cover. The question was: how can I replace a value in a comma separated string in a table?

This post covers the basics of this task.

Incidentally, this is where I say hey, that sounds like a failure in normalization. If you need to care about individual values in a collection, your value is not atomic. But that’s a bit of a tangent.

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Correlated Subqueries which Don’t

Published 2020-12-21 by Kevin Feasel

Daniel Hutmacher gives us an eye test:

The developer wrote this pretty little query to show us which accounts are up for review (which in our case means they have a “30” flag).

SELECT account, balance, 'For review' AS [status]
FROM #accounts WHERE account IN (SELECT account FROM #accountFlags WHERE flag=30) ORDER BY account;

Did you spot it?

I did, but in fairness, I’ve been burned enough times by this that I check for it.

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T-SQL Additions to Serverless SQL Pools

Published 2020-12-21 by Kevin Feasel

Jovan Popvic lays out some of the T-SQL syntax added to serverless SQL pools in Azure Synapse Analytics:

Serverless Synapse SQL pools in Azure Synapse Analytics have a new set of features that will enable you to analyze your Azure data more efficiently. The new Transact-SQL (T-SQL) language features that you can use in serverless SQL pools are STRING_AGG, OFFSET/FETCH, PIVOT/UNPIVOT, SESSION_CONTEXT, and CONTEXT_INFO.

Old T-SQL hands will likely know what all of this does, but click through if something looks unfamiliar. All of this is available in SQL Server 2017 and later (and everything but STRING_AGG() is available going back to 2008).

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A Use Case for Recursive CTEs

Published 2020-12-17 by Kevin Feasel

Jeffin Mathew takes us through a use case for recursive common table expressions:

An individual is working in HR and wants to find out which individual is managing who. This may be for several reasons such as, they need to ask the managers on the progress of their staff and if their appraisal is coming up or is due.

Another scenario may be that the company is enrolling more staff and wants to find out the capacity of the current staff or find individuals who have not yet got anyone to manage to give them the opportunity to do so.

Click through for the solution. Often times, we see recursive CTEs show up in hierarchical queries like this. When the number of records is small, they work really well. The issue comes with scale; that’s when a different table design becomes important.

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Coalesce in SQL and R

Published 2020-12-15 by Kevin Feasel

John MacKintosh gives us a primer on the COALESCE function in both SQL and R:

What does coalesce mean? In the English language, it is generally used to convey a coming together, or creating one whole body, mass or system. How does that help us when working with data? We spend a lot of time cleaning our data, surely the last thing we want to do is lump it all together?

Click through for detail on the nuances of COALESCE(). H/T R-Bloggers.

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Thoughts on R’s New Pipe

Published 2020-12-09 by Kevin Feasel

John Mount has thoughts on the upcoming pipe operator in R:

There is a current active discussion on this prototype and some interesting points come up. Note the current proposal appears to disallow a |> f -> f(a), a currently popular transform.

1. This is a language feature presented as a soon-to-be-user-visible prototype, not an RFC.
2. Some are objecting to the term “pipe.”
3. Some call this sort of pipe function composition.
4. It is noticed that this sort of substitution is generally thought of as a “macro.”
5. There is a claim the proposed pipe seems to violate the beta-reduction rule of the lambda calculus: variables should be substitutable for values.

Read on for John’s take on this. I particularly appreciate his response to point number 2: other functional languages have pipes (in fact, |> is the F# pipe operator). Pipes are not unique to UNIX. John has a lot of interesting comments, so check them out.

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First Thoughts on Amazon Babelfish

Published 2020-12-09 by Kevin Feasel

Ryan Booz shares some first thoughts on Amazon’s Babelfish offering:

The imputes for creating the tool is clear for AWS. Provide a way for customers to easily connect a SQL Server app to Aurora Postgres, saving big on licensing fees and reducing total cost of ownership. Assuming the tool is successful at some level, I’m sure it will provide a revenue boost for Amazon and some customers might (initially) feel a win. No harm, no foul on Amazon for leading the effort. Free markets, baby!

No matter how clever Babelfish is, however, I just can’t see how this is ultimately a win for SQL Server or PostgreSQL… or the developers that will ultimately need to support these “hybrid” apps.

I think Ryan makes good points and does hit upon the crux of the problem. I’d also say that there’s a secondary problem which Ryan hints at, but it is that a query may be sufficiently fast in one database variant but perform horridly in another. A classic example of this is a solution built on cursors in Oracle and then bringing that to T-SQL.

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