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Category: T-SQL

The REVERSE Function

Published 2017-06-08 by Kevin Feasel

John Morehouse explains what the REVERSE function is and why it can be useful:

Recently I had a conversation with a colleague that wasn’t aware of the REVERSE function in SQL Server.  I had sent them a code snippet and they inquired as to what the purpose of the function was.  Essentially, this function that reverses any string value.

For example, the phrase:

The brown dog jumped over the lazy fox

reversed looks like this

xof yzal eht revo depmuj god nworb ehT

Read on to see a good example of when you might use REVERSE.

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Check Those Aliases

Published 2017-06-02 by Kevin Feasel

Erik Darling warns you about accidentally using the wrong alias in a query:

People will often tell you to clearly alias your tables, and they’re right. It will make them more readable and understandable to whomever has to read your code next, puzzling over the 52 self joins and WHERE clause that starts off with 1 = 2. It can also help solve odd performance problems.

Take this query, for instance.

This isn’t just for subqueries; even simple joins can go haywire when you accidentally use the wrong alias and both tables happen to have the same column name.

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Building Newlines In SQL Scripts

Published 2017-06-01 by Kevin Feasel

Shane O’Neill engages in wacky newline misadventures:

GO is a special little guy. It’s not exactly T-SQL. It’s a way of telling the SQL Server Management Studio (SSMS) to send everything before it, from the beginning of the script or the preceding GO, to the SQL Server instance.

If you read the documents, the main point to take away is…

A Transact-SQL statement cannot occupy the same line as a GO command. However, the line can contain comments.

The problem turns out to be a little trickier than you’d first imagine.  Also, after reading this post, I think it’s lunchtime…

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Fisher’s Exact Test

Published 2017-05-25 by Kevin Feasel

Mala Mahadevan explains Fisher’s Exact Test and provides examples in T-SQL and R:

The decision rule in two sample tests of hypothesis depends on three factors :
1 Whether the test is upper, lower or two tailed (meaning the comparison is greater, lesser or both sides of gender and speaker count)
2 The level of significance or degree of accuracy needed,
3 The form of test statistic.
Our test here is to just find out if gender and speaker count are related so it is a two tailed test. The level of significance we can use is the most commonly used 95% which is also the default in R for Fischer’s Test. The form of the test statistic is P value. So our decision rule would be that gender and speaker category are related if P value is less than 0.05.

Click through for the R code followed by a code sample which should explain why you don’t want to do it in T-SQL.

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Generating Comma-Delimited Strings

Published 2017-05-25 by Kevin Feasel

Shane O’Neill has a good reason to upgrade to SQL Server 2017, which is not having to deal with FOR XML PATH hacks anymore:

Commas are all the rage nowadays:

There are a fair number of questions nowadays about returning data from a database in a comma separated string. Sure the application should probably do that but hey, database servers are expensive, why not get some bang for your bucks!

Protip:  I use the colon as a separator because sometimes commas will get stuck in the Ethernet cable.

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Understanding CROSS APPLY

Published 2017-05-25 by Kevin Feasel

Andy Levy has a T-SQL programming breakthrough:

Finally, this week I had a breakthrough. I was working on updating a bunch of data but it was breaking on a small subset of that data. In this case, I was attempting to JOIN two tables on fields that should have been INTs, but in a very small number of cases one side was using a comma-delimited string. The user told me that someone else had done these updates in the past and didn’t encounter the problem I was having (so I knew that it was something i was doing “wrong”), but given that it was only a handful of broken updates she was OK with manually doing the updates (we were scripting it because we were updating potentially tens of thousands of records).

I am not OK with manually fixing this in the future. I wanted to know how the other DBA had done it before. I dug into some history and found CROSS APPLY. My nemesis. I was determined to figure out how to use it this time.

The APPLY operator is extremely powerful in the right set of circumstances.  Andy shows the “classic” use case, but there are a number of other uses for the operator.

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T-SQL FizzBuzz Golf

Published 2017-05-23 by Kevin Feasel

Kenneth Fisher plays the FizzBuzz game with T-SQL and his chat piles on:

Now normally FizzBuzz is done with a loop, but as Russ said, we are using T-SQL so batch code is always the goal. That said, what table should I query to get the numbers 1-100? Well, I decided I’d just do something simple here and use a system view that’s more than 100 rows, the ROW_NUMBER function, and restrict it using TOP.

Read on to see several answers to this problem, some better than others.

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SQL Server Query Options

Published 2017-05-18 by Kevin Feasel

John Morehouse shows how to set your query options to be the same as that problematic application which is generating nasty queries:

Have you every executed a query in SQL Server Management Studio, looked at the execution plan, and noticed that it was a different plan than what was generated on the server?

A potential reason for this could be a different option settings.  The options represent the SET values of the current session.  SET options can affect how the query is execute thus having a different execution plan.   You can find these options in two places within SSMS under Tools -> Options -> Query Execution -> SQL Server -> Advanced.

Click through for lots of information including a script John provided to see which options are currently on.

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Creating Graph Objects In SQL Server

Published 2017-05-15 by Kevin Feasel

Steve Jones creates a simple graph relationship in SQL Server 2017:

What does all that mean? No idea. Clearly there is JSON that’s returned here and can be deserialized to gather meanings. Is this useful? I think graphs solve a certain set of problems very well, and more efficiently than relational systems. Certainly I could implement a graph structure relationally, but at scale I’m not sure the queries would be as easy to write or run as quickly.

I don’t know if I’d use a graph structure in any of the problems we try to solve in the SQLServerCentral app, but who knows. Maybe we would if we could.

Steve leaves this with more questions than answers, but he does give a very simple repro script if you want to futz about with graphs.

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