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Category: Syntax

MERGE in Dedicated SQL Pools

Emily Tehrani notes an addition to Azure Synapse Analytics:

We are thrilled to announce that the MERGE T-SQL command for Azure Synapse Dedicated SQL pools is now Generally Available! MERGE has been a highly requested addition to the Synapse T-SQL library that encapsulates INSERTs/UPDATEs/DELETEs into a single statement, drastically easing migrations and making table synchronization a breeze.

If you do decide to use this, I’d expect it to have the same bugs which make its use on-premises a mess. As always, MERGE responsibly.

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Deleting Rows in Order

David Fowler understand the order of things:

This was an interesting question that I was asked yesterday and something that I’d never really thought of before. Can you delete the top x number of rows based on an ORDER BY?

Why would you want to do that? Well let’s just assume that we have a ‘people’ table and some strange bug in the application has cause the need to delete the top 10 oldest females for whatever reason (I know, it’s very contrived example and not the situation that my colleague was facing but it’ll do to illustrate the point).

Click through for one way which doesn’t work and two ways which do.

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IS [NOT] DISTINCT FROM

Louis Davidson likes a new operator in SQL Server 2022:

The MOST exciting change from a T-SQL standpoint is: IS NOT DISTINCT FROM. This feature solves an age-old issue for T-SQL programmers and is worth its weight in gold. It is basically an equals comparison operator like =, but treats NULL as an individual value. Unlike =, this new operator returns only TRUE or FALSE, but not UNKNOWN. Writing queries that compare to values that can contain NULL is tedious, mostly because of code like the following:

Louis is quite happy here. I like the fact that the syntax is here, though I’d be concerned about performance—the syntax is nicer but you can run into the same performance issues as you’d have with “NULL or match” type queries.

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Compacting Window Function Definitions

Rob Farley like a syntax change:

This was fine, but it did start to become a little cumbersome.

Enter SQL Server 2022. Not only do we get the ability to ignore nulls now, making it easy to get the last non-null value from a list, but we also get a WINDOW clause – part of the SELECT query itself, dropping in between the HAVING clause and the ORDER BY clause, allowing us to predefine those OVER clause segments.

Read on for the full scope of Rob’s thoughts.

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Working with CROSS APPLY

Paul Randal takes us through one of my favorite operators:

Introduced by Microsoft in SQL Server 2005, SQL CROSS APPLY allows values to be passed from a table or view into a user-defined function or subquery. This tutorial will cover the incredibly useful and flexible APPLY operator, such as how the CROSS APPLY and OUTER APPLY operators work, how they’re like the INNER and LEFT OUTER JOIN, and give you some examples of both. All the examples use the AdventureWorks example database.

Later in the article, I’ll also discuss a highly pervasive SQL Server performance problem—one I still encounter with customers on a weekly basis. This problem is related to using the APPLY operator against a specific type of user-defined function. It’s so problematic and can overwhelm tempdb, bringing your SQL Server instance to a crawl!

If you’re not too familiar with APPLY in its two forms, read the whole thing.

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Building UNPIVOT Syntax

Michael J. Swart has a function:

Just like PIVOT syntax, UNPIVOT syntax is hard to remember.
When I can, I prefer to pivot and unpivot in the application, but here’s a function I use sometimes when I want don’t want to scroll horizontally in SSMS.

Click through for the function. This is an area where I wish there was built-in * logic for PIVOT and UNPIVOT. Or at least a “Select all columns but the following” as that would make things easier.

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Improvements to GENERATE_SERIES

Erik Darling notes some improvements:

With the release of CTP 2.1, the problems that we saw the first time around are all gone. But there are still a couple small caveats that you should be aware of.

There’s also been a change in the way you call the function, too. You not longer need the START and STOP identifiers in the function.

There are still some limitations but it does look like the function is considerably better in CTP 2.1.

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T-SQL Improvements in CTP 2.1

Itzik Ben-Gan looks at ways recent T-SQL improvements:

One of the complexities that is of a specific importance to this section is comparisons that potentially involve NULL comparands, such as ones that you use in filter and join predicates. Most operators that you use in such comparisons, including the equals (=) and different than (<>) operators, use three-valued logic. This means that there are three possible truth values as a result of a predicate that uses such operators: true, false and unknown. When both comparands are non-NULL, such operators return true or false as you would intuitively expect. When any of the comparands is NULL, including when both are NULL, such comparisons return the unknown truth value. Both filter and join predicates consider unknown as a non-match, and sometimes that’s not the behavior that you’re after.

The distinct predicate (IS [NOT] DISTINCT FROM) should simplify a lot of code in the wild.

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CETAS and the Serverless SQL Pool

Liliam Leme takes us through the Create External Table as Select operation in the Azure Synapse Analytics serverless SQL pool:

Serverless SQL pool  has a very cool way to handle CREATE EXTERNAL TABLE AS SELECT (CETAS).  I mean, you can use a serverless SQL pool to create a CETAS which will materialize the query results. It means a heavy query in which, for example, the results would be part of future large join with other queries or aggregations that can be consolidated for reporting purposes. Those scenarios are examples that could be used as a CETAS. CETAS will be stored as a parquet file accessed by an external table in your storage and the performance is awesome. Later I am suggesting how to recreate the CETAS using pipeline against serverless SQL Pool.

Click through to see it in action.

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