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Category: T-SQL

Updating Multiple Tables In One Statement

Published 2017-10-27 by Kevin Feasel

Denis Gobo tries different methods of updating two tables in one statement:

Every now and then you will get someone asking how to update two tables with one statement in SQL Server. The answer is usually, no that is not possible… the person then walks away muttering something about how MySQL allows it.

So I decided to try to see if I could update two tables with one statement. I decided to try a couple of different things

  • view

  • common table expression

  • indexed view

  • instead of trigger

The results are generally negative.  I get the benefit of updating multiple tables in a single query, particularly when you have fairly complex logic around determining which records to update, but if you just wrap things in a transaction, at least you get the benefits of atomicity.  And if you often have to update a bunch of different tables at the same time, there might be an issue with your data model.

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Comparing Ranking Functions

Published 2017-10-20 by Kevin Feasel

Doug Kline compares three window functions:  RANK, DENSE_RANK, and ROW_NUMBER:

— so let’s say that we’ve created a contest

— places in the contest (top place, 2nd place, etc.)
— will be determined by the test score

— in other words, we’re not so concerned with the raw score
— but rather, we’re interested in the *relative* score
— and the order in which people appear, based on their score

— we can use the ROW_NUMBER() function to give a
— ‘ranking’ to each record, based on Score

Doug’s post is a video and an extended script so you can follow along.

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Updating Data In Common Table Expressions

Published 2017-10-20 by Kevin Feasel

Kenneth Fisher shows that you can directly update a table referenced in a common table expression:

CTEs are cool things. You can essentially create one or more in-line view(s) within your query. One thing that isn’t overly well known is that you can actually update the data within the CTE. No, I don’t mean using using the UPDATE statement with a CTE but actually running the update through the CTE.

This is really powerful when combined with window functions, like only updating the first record given a particular partition.  You can also delete, which makes duplicate detection and deletion fairly straightforward.

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Default Schemas In SQL Server

Published 2017-10-13 by Kevin Feasel

Daniel Hutmacher looks at specifying default schemas on a database:

If your user is a database owner, (i.e. is a member of the db_owner group or has CONTROL permissions on the database) the default schema will always be dbo. This is something you can’t change.

So if your legacy application needs quasi-administrative privileges in the database, you can’t make it a database owner, but you can grant those permissions on the schema instead (which is actually a better idea anyway).

What Daniel is doing is akin to the pre-2005 concept of user spaces, where Bob had a schema and Mary had a schema and Jill had a schema and so forth.

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System Objects And Helper Functions

Published 2017-10-12 by Kevin Feasel

Kenneth Fisher compares and contrasts querying system tables versus using built-in helper functions which query the system tables:

So which should you use? Well, like most things in the database world, it depends. Do you need to cross databases? Do you need to use NOLOCK for a diagnostic query? Do you need additional information available in one of the views that isn’t available in one of the helper functions?

Click through for the comparison.  I agree with his end result, that you should at least know both methods, even if you lean toward one.

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COALESCE, ISNULL, And Data Types

Published 2017-10-10 by Kevin Feasel

Shane O’Neill shows a subtle difference between ISNULL and COALESCE:

You may be asking yourself “What the…?!”.

I asked myself the same question but then I thought of a better idea and asked my Senior the question instead (he usually has better answers than I do), who proceeded to tell me that the final test would only work exactly at midnight!

….so I repeated my question to him.

It’s an interesting read, and not something you’d commonly think about.

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Sparklines In R

Published 2017-10-06 by Kevin Feasel

Robert Sheldon shows how to use SQL Server R Services to display sparklines for categories:

In this article, we continue our discussion on visualizations, but switch the focus to sparklines and other spark graphs. As with many aspects of the R language, there are multiple options for generating spark graphs. For this article, we’ll focus on using the sparkTable package, which allows us to create spark graphs and build tables that incorporate those graphs directly, a common use case when working with spark images.

In the examples to follow, we’ll import the sparkTable package and generate several graphs, based on data retrieved from the AdventureWorks2014 sample database. We’ll also build a table that incorporates the SQL Server data along with the spark graphs. Note, however, that this article focuses specifically on working with the sparkTable package. If you are not familiar with how to build R scripts that incorporate SQL Server data, refer to the previous articles in this series. You should understand how to use the sp_execute_external_script stored procedure to retrieve SQL Server data and run R scripts before diving into this article.

Sparklines and associated visuals have their place in the world.  Read on to see how you can build a report displaying them.

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Finding Progress On A Long-Running Statement

Published 2017-10-06 by Kevin Feasel

David Fowler shows us how to track how far we’ve gotten on a long-running data modification statement:

Well, it would turn out that there is and to find out we need to turn to our trusty friend, the transaction log.

As we know, the transaction log will squirrel away an entry each time that a row is modified.  We can count up all the LOP_MODIFY_ROW, LOP_INSERT_ROW and LOP_DELETE_ROWS entries for our transaction and that will tell us just how many rows our transaction has altered so far.

Click through for a script, as well as an important disclaimer.

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Finding Broken Views

Published 2017-10-06 by Kevin Feasel

Bill Fellows has a script to test each view to see if it is broken:

Shh, shhhhhh, we’re being very very quiet, we’re hunting broken views. Recently, we were asked to migrate some code changes and after doing so, the requesting team told us we had broken all of their views, but they couldn’t tell us what was broken, just that everything was. After a quick rollback to snapshot, thank you Red Gate SQL Compare, I thought it’d be enlightening to see whether anything was broken before our code had been deployed.

You’ll never guess what we discovered.

Read on to see what they discovered (spoilers:  broken views) and how Bill fixed the problem.

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Why Window Functions Can’t Appear In The WHERE Clause

Published 2017-10-03 by Kevin Feasel

Doug Lane explains why window functions like ROW_NUMBER() cannot appear in the WHERE clause (or FROM, GROUP BY, or HAVING):

SQL Server doesn’t process parts of a query in the same order they’re written. Rather than start with SELECT the way we read and write it, here’s the order SQL Server progresses through:

  1. FROM
  2. WHERE
  3. GROUP BY
  4. HAVING
  5. SELECT
  6. ORDER BY
  7. TOP

The first four steps are all about getting the source data and reducing the result set down. Steps 5 & 6 determine which columns are presented and in which order. Step 7 (TOP) is only applied at the end because you can’t say which rows are in the top n rows until the set has been sorted. (You can read Itzik Ben-Gan’s explanation of this process in way more detail here.)

Definitely worth reading.  Doug also shows how to get around this fact of life and get the equivalent of a window function inside a WHERE clause, at least in terms of function if not necessarily performance.

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