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Category: T-SQL

Comparing Distinctness

Published 2018-03-28 by Kevin Feasel

Michael J. Swart shows several options for comparing whether an attribute’s value is distinct from a parameter:

Check it:

DECLARE @TeamId bigint = NULL,
    @SubTeamId bigint = NULL;
 
SELECT TOP 1 TaskId
FROM tasks
WHERE assignedTeamId IS NOT DISTINCT FROM @TeamId
  AND assignedSubTeamId IS NOT DISTINCT FROM @SubTeamId

Talk about elegant! That’s what we wanted from the beginning. It’s part of ANSI’s SQL 1999 standard. Paul White tells us it’s implemented internally as part of the query processor, but it’s not part of T-SQL! There’s a connect item for it… err. Or whatever they’re calling it these days. Go read all the comments and then give it a vote. There are lots of examples of problems that this feature would solve.

PROS: Super-elegant!
CONS: Invalid syntax (vote to have it included).

This would be nice to have.  In the meantime, Michael shows several options which are currently valid syntax.

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Using SQL Server’s PIVOT Operator

Published 2018-03-26 by Kevin Feasel

Dan Blank shows how to use the PIVOT operator in SQL Server:

I was recently approached by my firms Marketing Manager with a request for some information.  She wanted to know “Which departments have our top clients never done any work for?”.  For some clarity, I work in a law firm with 11 departments. The request seems pretty straightforward at first. Then once I got thinking about how the output of this report would be presented it made me reconsider quite how simple a request this was.  She handed me a drawing with her vision for the output.  What she wanted was :

  1. Client’s names down the left,

  2. List of Departments across the top,

  3. Ticks and crosses at the intersection to show whether they had or had not done work for them.

You can make a good argument that presentation mechanics like this are meant for a different tool (a presentation layer) but it’s useful to know how to pivot and unpivot data within T-SQL for more reasons than just presentation.

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The Secret Power Of TRIM

Published 2018-03-26 by Kevin Feasel

Dave Mason points out something quite useful about TRIM in SQL Server 2017:

Now I can tidy things up and remove both leading and trailing spaces with a single call to TRIM():

SELECT TRIM(b.foo)
FROM dbo.bar b;

On the surface, this may not seem like that big of a deal. And I would tend to agree. But, in this example, it saves a few key strokes and makes the code slightly more readable. And it is nice for T-SQL to finally have a function that has been around in other languages for far longer than I’ve been writing code for a living.

But Wait, There’s More!

Click through for that more.  This makes TRIM a lot more useful, so go check it out.

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ORIGINAL_DB_NAME()

Published 2018-03-16 by Kevin Feasel

Kenneth Fisher explains a couple of database name functions in SQL Server:

I’d never seen ORIGINAL_DB_NAME until recently and I thought it would be interesting to highlight it out, and in particular the difference between it and DB_NAME. I use DB_NAME and DB_ID fairly frequently in support queries (for example what database context is a query running from or what database are given DB files from). So starting with DB_NAME.

Click through to know when to use each.

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Using STRING_AGG In SQL Server 2017

Published 2018-03-09 by Kevin Feasel

Derik Hammer talks about one of the nicer T-SQL additions in SQL Server 2017:

Creating comma separated strings from a column, or delimited strings as I like to call it, is a very common problem in SQL. Beginning with SQL Server 2017 and Azure SQL Database, there is now another option to the existing set of solutions, STRING_AGG().

I would like to convince you to use STRING_AGG over the other methods. So, let us begin with the competing solutions.

I completely agree and have been switching code over to use STRING_AGG since upgrading to 2017.  The code is so much clearer as a result compared to STUFF + FOR XML PATH concatenation.

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Getting A Random Row

Published 2018-03-06 by Kevin Feasel

Brent Ozar shares four methods for getting a random row from a table:

Method 1, Bad: ORDER BY NEWID()

Easy to write, but it performs like hot, hot garbage because it scans the entire clustered index, calculating NEWID() on every row:

That took 6 seconds on my machine, going parallel across multiple threads, using tens of seconds of CPU for all that computing and sorting. (And the Users table isn’t even 1GB.)

Click through for the other three methods.  The really tricky part is when you want to get a random sample from the table, as TABLESAMPLE is an awful choice for that.

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Gapless Sequential Values

Published 2018-02-28 by Kevin Feasel

Gail Shaw shows how to build gapless sequential values:

To be clear, I don’t think this is a good idea. The identity column works well if a sequential series of numbers are needed. If the number sequence needs to that spans tables, then the sequence object is a good replacement.

But, there will always be some requirements that insist on gap-less sequences, or insist on not using identity (probably for ‘compatibility’ reasons), so let’s see how to do it properly.

For true surrogate keys, this is a bad idea because it is both unnecessary and overkill.  Where this becomes useful is cases where an auditor is expecting a proper sequence without any gaps, such as invoice numbers, check numbers, or purchase order numbers.  As Gail mentions, those are uncommon scenarios.

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Special Commands In mssql-cli

Published 2018-02-27 by Kevin Feasel

Alan Yu announces an update to mssql-cli:

Special commands are meant to make your life easier. They are shortcuts to perform common tasks and queries. All special commands start with a backslash (\), and you can use the built-in IntelliSense to see a list of special commands you can use. You can also learn more by running the following command:

$ mssql-cli>\?

To start off, let’s say you have a common query you run often, but don’t want to type it out each time or scroll through your history. You can “save” your query by running the following special command:

$ mssql-cli>\sn address select * from "Person"."AddressType"

To execute this query, simply run:

$ mssql-cli>\n address

Click through to learn more and see it in action.

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The Value Of Schemabinding

Published 2018-02-15 by Kevin Feasel

Vitaly Bruk explains what schemabinding is and why we sometimes need WITH SCHEMABINDING in our code:

In SQL Server, when we use the “WITH SCHEMABINDING” clause in the definition of an object (view or function), we bind the object to the schema of all the underlying tables and views. This means that the underlying tables and views cannot be modified in a way that would affect the definition of the schema-bound object. It also means that the underlying objects cannot be dropped. We can still modify those tables or views, as longs as we don’t affect the definition of the schema-bound object (view or function).

If we reference a view or function in a schema-bound object (view or function), then the underlying view or function must also be schema-bound. And we cannot change the collation of a database, if it contains schema-bound objects.

I’ve only used schemabinding when mandated (e.g., using row-level security or creating an indexed view), but I can see the value behind using it with normal development.

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GROUP BY vs DISTINCT

Published 2018-02-15 by Kevin Feasel

Rob Farley looks at how GROUP BY and DISTINCT and lead you down different execution plan paths:

What I want to explore in this post is the particular example that we both used… to bring an important point that could be missed because of the similarity of our examples.

You see, we both happened to use a FOR XML concatenation query, looking back at the same table. We did this to simulate a practical GROUP BY – somewhere that you might feel like GROUP BY is useful, but you know that you’re not using an aggregate function like SUM or MAX, but there isn’t one available. Ok, for Aaron he could’ve used the really new STRING_AGG, but for the old-timer like me, having to use SQL Server 2005, that wasn’t available.

In this post, Rob looks at a different sort of example and sees a more complicated scenario unfold.

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