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Curated SQL Posts

Dealing With The Registry From SQL Server

Published 2017-08-17 by Kevin Feasel

Wayne Sheffield shows how to read and modify registry entries using SQL Server:

xp_instance_regread

In this example, I used xp_regread to read the direct registry path. If you remember from earlier, there are SQL Server instance-aware versions of each registry procedure. A comparable statement using the instance-aware procedure would be:

This statement returns the exact same information. Let’s look at the difference between these – in the first query, the registry path is the exact registry path needed, and it includes “\Microsoft SQL Server\MSSQL12.SQL2014\”. In the latter query, this string is replaced with “\MSSQLSERVER\”. Since the latter function is instance aware, it replaces the “MSSQLSERVER” with the exact registry path necessary for this instance of SQL Server. Pretty neat, isn’t it? This allows you to have a script that will run properly regardless of the instance that it is being run on. The rest of the examples in this post will utilize the instance-aware procedures to make it easier for you to follow along and run these yourself.

Sometimes you just have to change something in the registry from SQL Server.  Hopefully that “sometimes” is rare.

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Attaching Databases To Docker

Published 2017-08-17 by Kevin Feasel

Andrew Pruski shows one scenario where Docker on Windows is better than Docker on Linux:

One of the (if not the) main benefits of working with SQL in a container is that you can create a custom image to build container from that has all of your development databases available as soon as the container comes online.

This is really simple to do with Windows containers. Say I want to attach DatabaseA that has one data file (DatabaseA.mdf) and a log file (DatabaseA_log.ldf): –

ENV attach_dbs="[{'dbName':'DatabaseA','dbFiles':['C:\\SQLServer\\DatabaseA.mdf','C:\\SQLServer\\DatabaseA_log.ldf']}]"

Nice and simple! One line of code and any containers spun up from the image this dockerfile creates will have DatabaseA ready to go.

However this functionality is not available when working with Linux containers. Currently you cannot use an environment variable to attach a database to a SQL instance running in a Linux container.

Read on to see what you can do if you’re using a Linux container.

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NULL Replacement In SQL Server And Oracle

Published 2017-08-17 by Kevin Feasel

Daniel Janik shows a pair of non-standard functions you can use to replace NULL values:

It’s Wednesday and that means another SQL/Oracle post. Today we’ll be discussing NULL Values, which can sometimes be a real pain. Don’t worry though there’s a simple solution. Simply replace the NULL value with another.

Comparing a column with NULL and replacing with another value is really simple. There are built in functions for replacing NULL values. I’m not going to discuss the ANSI standard COALESCE here. If you want to know more about it you can find it on Bing.

I provide no comment on Daniel’s claim regarding being able to find something on Bing…  Click through to see the custom NULL replacement functions in SQL Server versus Oracle.

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Biml Enrichment With Annotations

Published 2017-08-17 by Kevin Feasel

Bill Fellows shows why it’s useful to include annotations in your Biml scripts:

In many of the walkthroughs on creating relational objects via Biml, it seems like people skim over the Databases collection. There’s nothing built into the language to really support the creation of database nodes. The import database operations are focused on tables and schemas and assume the database node(s) have been created. I hate assumptions.

Read on for more about dealing with databases, and not just tables and other database objects, in Biml.

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Dealing With Limited Rights In Biml

Published 2017-08-17 by Kevin Feasel

Shannon Lowder walks through a scenario where he wants limited rights to process metadata changes, separate from any data transfer:

My development environment has a local instance of SQL Server with AdventureWorks2014 on it.  I’m going to use that as my source.  I also created a database on this instance called BimlExtract to serve as my destination database.

To create a user that can only read the schema on the source system, I created a login and user named ‘Biml’.  I granted this user VIEW DEFINITION in AdventureWorks2014. I also added this user to the db_owner group in BimlExtract.  Now, this user can read the schema of the source, and create tables in the destination. I’ve included the T-SQL to set the permissions in Database Setup.sql.

Now, we’re ready to walk through the solution.

Click through for the solution and also a GitHub repo with all of Shannon’s code.

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Understanding Decision Trees

Published 2017-08-16 by Kevin Feasel

Ramandeep Kaur explains how decision trees work:

Simply put, a decision tree is a tree in which each branch node represents a choice between a number of alternatives, and each leaf node represents a decision.

It is a type of supervised learning algorithm (having a pre-defined target variable) that is mostly used in classification problems and works for both categorical and continuous input and output variables. It is one of the most widely used and practical methods for Inductive Inference. (Inductive inference is the process of reaching a general conclusion from specific examples.)

Decision trees learn and train itself from given examples and predict for unseen examples.

Click through for an example of implementing the ID3 algorithm and generating a decision tree from a data set.

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Bad Parameter Sniffing Flowchart

Published 2017-08-16 by Kevin Feasel

Grant Fritchey is asking for input on a new flowchart he has created:

Lots of people are confused by how to deal with bad parameter sniffing when it occurs. In an effort to help with this, I’m going to try to make a decision flow chart to walk you through the process. This is a rough, quite rough, first draft.

I would love to hear any input. For this draft, I won’t address the things I think I’ve left out. I want to see what you think of the decision flow and what you think might need to be included. Click on it to embiggen.

I think it’s a great first step.  I think a decision to add local variables and use them instead of parameters would be useful, particularly in contrast to using RECOMPILE and OPTIMIZE FOR UNKNOWN.

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Checkpointing Code For Reproduction

Published 2017-08-16 by Kevin Feasel

David Smith tells an interesting story about a reproducibility problem with data analysis:

Timo Grossenbacher, data journalist with Swiss Radio and TV in Zurich, had a bit of a surprise when he attempted to recreate the results of one of the R Markdown scripts published by SRF Data to accompany their data journalism story about vested interests of Swiss members of parliament. Upon re-running the analysis in R last week, Timo was surprised when the results differed from those published in August 2015. There was no change to the R scripts or data in the intervening two-year period, so what caused the results to be different?

The version of R Timo was using had been updated, but that wasn’t the root cause of the problem. What had also changed was the version of the dplyr package used by the script: version 0.5.0 now, versus version 0.4.2 then. For some unknown reason, a change in the dplyr package in the intervening package caused some data rows (shown in red above) to be deleted during the data preparation process, and so the results changed.

Click through for the solution, which is pretty easy in R.

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Memory-Optimized Object Isolation Levels

Published 2017-08-16 by Kevin Feasel

Ned Otter looks at the isolation levels offered when you work with memory-optimized objects:

If you are only querying on-disk tables, you can use any of the isolations levels from List 1. And if you are only querying memory-optimized tables, you can use any of the isolation levels from List 2.

But what if you want to reference both on-disk and memory-optimized tables in the same query? Of course, the answer is “it depends”, with transaction initiation modes and isolation levels being the components of that dependency.

This post is part one of a series and is mostly around level-setting.

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What Is DAX?

Published 2017-08-16 by Kevin Feasel

Matt Allington covers some of the basics of DAX:

Do I need to learn the DAX language?

You certainly do not need to know how to write DAX to get started with Power BI.  Power BI is the newest business intelligence tool that leverages the DAX language (via Power Pivot) and it is definitely possible to get started and build some reports without learning any DAX at all.   If you are a “consumer of reports” that other people produce for you then you certainly don’t need to learn any DAX.  However if you are someone that wants to do your own adhoc (or structured) analysis of data using Power BI, Power Pivot for Excel, then you will definitely want to learn to write some DAX in order to get value from what these new tools have to offer.

It’s a good intro if you aren’t familiar with DAX.

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