Press "Enter" to skip to content

Category: T-SQL

Shred That XML

Published 2016-09-30 by Kevin Feasel

Steve Jones has an intro-level post on shredding an extended event to get to the relevant portion:

I was playing with some Extended Events recently. If you haven’t tried, I’d encourage you to do so. However, working with XML is not my favorite. I know I can get the GUI in SSMS 16.x to show me events, but I sometimes want to query.

Here was my quick adventure in XML and XQUERY. I should know this stuff better, but I think I’m working with XML so rarely that I’m constantly re-learning things.

Read on for the code.

Comments closed

Don’t Use Double Dot

Published 2016-09-27 by Kevin Feasel

Chris Bell warns against using double dot syntax:

I am finding more and more cases where SQL code is being created using the double dot or period for the 2 part naming convention.

For example, instead of using dbo.table1 I am seeing ..table1.

I don’t know who suggested this in the first place, but it is not a good idea. Sure it works and does what you expect, but there is a HUGE risk with doing this. When you use the .. syntax, you are telling the code to use whatever the default schema is for the user that is running the query. By default that is the dbo schema, but there is no guarantee that all systems are going to be that way.

Read on to understand why this is a big deal.

Comments closed

One-Sample T Tests

Published 2016-09-21 by Kevin Feasel

Mala Mahadevan shows how to perform one-sample T Tests:

For this post I decided to go with a simple example of how many steps I walked with my per day for the month of August. My goal is 10,000 steps per day – that has been my average over the year but is this true of the data I gathered in August? I have a simple table with two columns – day and steps. Each record has how many steps I took in August per day, for 30 days. So – SELECT AVG(steps) FROM [dbo].[mala-steps] gives me 8262 as my average number of steps per day in August. I want to know if am consistently under performing my goal, or if this is a result of my being less active in August alone. Let me state my problem first – or state what is called ‘null hypothesis’:

I walk 10,000 steps on an average per year. 

Read on for T test operations in T-SQL (although not all operations are available) and R.

Comments closed

Ordering In Views

Published 2016-09-20 by Kevin Feasel

Kenneth Fisher explains why you shouldn’t order in views:

For many years it’s been a best practice to never put an ORDER BY in a view. The idea is that a view shouldn’t have an inherent order. Just like any other query. If you want the data from a view ordered then you query the view with an ORDER BY clause. In fact if you put an ORDER BY in a view you’ll get an error:

Msg 1033, Level 15, State 1, Procedure MyView, Line 4 [Batch Start Line 2]
The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP, OFFSET or FOR XML is also specified.

I knew about the TOP 100 PERCENT bit, but had no idea that order was outright ignored.  Read the comments for additional information.

Comments closed

Chi Square Tests

Published 2016-09-14 by Kevin Feasel

Mala Mahadevan discusses how to perform a Chi Square test:

For any dataset to lend itself to the Chi Square test it has to fit the following conditions  –

1 Both  variables are categorical (in this case – exposure to smoking – yes/no, and health condition – sick/not sick are both categorical).
2 Researchers used a random sample to collect data.
3 Researchers had an adequate sample size.Generally the sample size should be at least 100.
4 The number of respondents in each cell should be at least 5.

This is an easy case for using R over T-SQL—the Chi Square test is built in, whereas you have to roll your own T-SQL code.  Mala does show you how to do this from within SQL Server R Services as well.

Comments closed

Finding Destinations In SSISDB

Published 2016-09-09 by Kevin Feasel

Bill Fellows has a script to figure out the name of that table throwing errors upon insertion:

There is a rich set of tables and views available in the SSISDB that operate as a flight recorder for SSIS packages as they execute. Markus Ehrenmüller (t) had a great question in Slack. In short, can you figure out what table is being used as a destination and I took a few minutes to slice through the tables to see if I could find it.

If it’s going to be anywhere, it looks like you can find it in catalog.event_message_context

If someone is using an OLE DB Destination and uses “Table or view” or “Table or View – fast load” settings, the name of the table will be the event message_context table. If they are using a variable name, then it’s going to be trickier.

Read on for the script.

Comments closed

Try-Catch Blocks

Published 2016-09-08 by Kevin Feasel

Dave Mason discusses a major problem with try-catch blocks:

We didn’t just lose one error. We lost them all! DBCC CHECKDB on a corrupt database just came back “clean” and the CATCH block never executed. We’ve moved into dangerous territory now. This is no longer an inconvenience or an annoyance. This could lead to data loss, financial harm to your company, maybe even some risk to your career.

Dave promises a better approach in his next post.

Comments closed

Pattern Matching

Published 2016-09-07 by Kevin Feasel

Aaron Bertrand has an article on performance testing various pattern matching techniques:

I wanted to prove to myself that the above procedure was undeniably better overall for all possible searches than any of the queries that use ORconditionals, regardless of whether they use LIKE, LEFT/RIGHT, SUBSTRING, or CHARINDEX. I took David’s basic query structures and put them in stored procedures (with the caveat that I can’t really test “contains” without his input, and that I had to make his OR logic a little more flexible to get the same number of rows), along with a version of my logic. I also planned to test the procedures with and without an index I would create on the search column, and under both a warm and a cold cache.

The conclusion is ambiguous.  Read on for the full story.

Comments closed

Pearson’s Correlation Coefficient

Published 2016-09-06 by Kevin Feasel

Mala Mahadevan explains correlation coefficients:

The statistical definition of Pearson’s R Coefficient, as it is called, can be found in detail here for those interested. A value of 1 indicates that there is a strong positive correlation(the two variables in question increase together), 0 indicates no correlation between them, and -1 indicates a strong negative correlation (the two variables decrease together). But you rarely get a perfect -1, 0 or 1. Most values are fractional and interpreted as follows:
High correlation: .5 to 1.0 or -0.5 to 1.0.
Medium correlation: .3 to .5 or -0.3 to .5.
Low correlation: .1 to .3 or -0.1 to -0.3.

Mala includes R and T-SQL code so you can follow along.

Comments closed