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Category: T-SQL

Dynamic Searches In SQL

Published 2017-02-28 by Kevin Feasel

Kenneth Fisher looks at a few methods for dynamic searches in T-SQL:

Multiple Queries

Pro: We get a separate query plan for each combination of parameters so performance is great. (Well, as good as can be anyway.)
Con: Maintance stinks. We need 16 different queries when we have 4 parameters and the numbers increase dramatically as we add additional parameters. So any change we make to the base query will have to be changed 16 times, and/or adding a new parameter means careful logic and adding a bunch of new queries.

I’d consider this a gateway for Erland Sommarskog’s article on the topic.

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Pattern Matching With LIKE

Published 2017-02-23 by Kevin Feasel

Shane O’Neill looks at different ways to match patterns with the LIKE operator:

So let’s test out this bad boy using the WideWorldImporters database, see if we can find everyone with the first name of Leyla.

Simple right? And because [Sales].[Customers] uses the full name, we have to use LIKE.

Now a developer comes along and says “Wait a second, my sister is Leila”. So we try to cheat and add a wildcard in there.

Leonardo!? Well I suppose he does count in this situation, but there’s 2 characters between the ‘e’ and the ‘a’ and I only wanted one.

Click through for a couple pattern matching tricks and look for ways to avoid Lejla in your life.

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Unpivoting Wide Tables

Published 2017-02-23 by Kevin Feasel

Dave Mason has to unpivot a particularly wide table:

Today’s less-than-ugent challenge was to un-pivot the output of RESTORE HEADERONLY. I thought for certain someone else, somewhere, at at some time must have wanted to do the same thing. So I asked the Twitterverse, but no one responded. I guess I’ll have to make do myself without the easy button. No worries, though. We can do this!

Metadata tables are good friends at times like these.

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Gaps And Islands: Call Center Edition

Published 2017-02-22 by Kevin Feasel

Kathy Kellenberger solves an issue with tracking contiguous calls within a call center:

To come up with a solution, I first thought about how to solve it with a traditional cursor. I won’t provide a cursor solution here, but it would involve looping through the rows in order and finding the first and last employee rows. When the last EmpID doesn’t match the next EmpID, then the previous row is the last call for the previous EmpID and the current row is the first call for the next EmpID.

NOTE: The solution to this problem assumes that there are at least two calls per shift.

In 2012, four T-SQL window functions became available that let you look at expressions from different rows: LAG, LEAD, FIRST_VALUE, and LAST_VALUE. For this puzzle, LAG and LEAD seemed to be the answer.  In this case, I used the optional default parameter to replace any NULLs with -1.

This is a particular solution for the Gaps and Islands problem; here’s Itzik Ben-Gan on the topic.

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SET NOEXEC ON

Published 2017-02-20 by Kevin Feasel

Andy Kelly shows how to prevent part of a script from running:

 

If we were to hit F5 (or however you execute your TSQL statements in SSMS) without highlighting any statement(s) they would all be executed, one batch after the other. Even if one batch were to fail or we had a THROW in that batch it would fail at that point but execution would continue immediately after the next GO until the end. This is where SET NOEXEC ON comes into play. If I add that at the beginning of the script all succeeding code would not be executed. The statements would only be compiled and not actually run. It would look like this:

SET NOEXEC ON;

PRINT ‘Got Here 1’ ;
GO
PRINT ‘Got Here 2’ ;
GO
PRINT ‘Got Here 3’ ;
GO
PRINT ‘Got Here 4’ ;
GO

This is a useful “accidental F5” protection:  you can put it at the top of your long script to keep from running the whole thing at once.

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String Splitting And Concatenation

Published 2017-02-16 by Kevin Feasel

Aaron Bertrand and Steve Hughes talk about string splitting, and Aaron also discusses string concatenation.  First Aaron:

That may not look like a massive simplification, but don’t forget about all the logic buried behind the table-valued function in the first example. And if you’re like several shops I know, if you look across your codebase and see all the messy uses you have for either of these methods, the benefits should be even more clear – and testing should bear that the performance savings compared to traditional, expensive methods are the sweetest part of the deal.

And Steve:

The STRING_SPLIT function will return a single column result set. The column name is “value”. The datatype will be NVARCHAR for strings that are NCHAR or NVARCHAR. VARCHAR is used for strings that are CHAR or VARCHAR types.

These two functions are small, but come in handy quite frequently.

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AT TIME ZONE And Reports

Published 2017-02-16 by Kevin Feasel

Rob Farley shows how to use AT TIME ZONE without sacrificing performance:

Because how am I supposed to know whether a particular date was before daylight saving started or after? I might know that an incident occurred at 6:30am in UTC, but is that 4:30pm in Melbourne or 5:30pm? Obviously I can consider which month it’s in, because I know that Melbourne observes daylight saving time from the first Sunday in October to the first Sunday in April, but then if there are customers in Brisbane, and Auckland, and Los Angeles, and Phoenix, and various places within Indiana, things get a lot more complicated.

To get around this, there were very few time zones in which SLAs could be defined for that company. It was just considered too hard to cater for more than that. A report could then be customised to say “Consider that on a particular date the time zone changed from X to Y”. It felt messy, but it worked. There was no need for anything to look up the Windows registry, and it basically just worked.

But these days, I would’ve done it differently.

Now, I would’ve used AT TIME ZONE.

Read on for the scenario.

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Session Context

Published 2017-02-16 by Kevin Feasel

Ewald Cress looks at SESSION_CONTEXT() as a replacement for CONTEXT_INFO():

SESSION_CONTEXT() brings two major innovations. Firstly, it replaces a 128-byte scalar payload with a key-value structure that can accommodate 256kB of data. You can really go to town filling this thing up.

The second change is less glamorous, but possibly more significant: it is possible to set an entry to read-only, meaning that it can safely be used for the kind of contextual payload you don’t want tampered with. This makes me happy, not because I currently have a great need for it, but because it neatly ties in with things I have been thinking about a lot lately.

Read on for more.

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Metaphones In SQL

Published 2017-02-15 by Kevin Feasel

Phil Factor builds a function to generate metaphones in SQL Server:

Metaphone algorithms are designed to produce an approximate phonetic representation, in ASCII, of regular “dictionary” words and names in English and some Latin-based languages. It is intended for indexing words by their English pronunciation. It is one of the more popular of the phonetic algorithms and was published by Lawrence Philips in 1990. A Metaphone is up to ten characters in length.

It is used for fuzzy searches for records where each string to be searched has an index with a Metaphone key. You search for all records with the same or similar metaphone key and then refine the search by some ranking algorithm such as Damerau–Levenshtein distance. Metaphone searches are particularly popular with ‘ancestor’ sites that search on surnames where spellings vary considerably for the same surname. The current version, Metaphone 3, is actively maintained by Lawrence Philips, developed to account for all spelling variations commonly found in English words, first and last names found in the United States and Europe, and non-English words whose native pronunciations are familiar to English-speakers. The source of Metaphone 3 is proprietary, and Lawrence charges a fee to supply the source.

Read on for the script.

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Generating SQL Permutations

Published 2017-02-15 by Kevin Feasel

Michael J. Swart uses a recursive common table expression and bit shifting to build a full set of permutations:

If you google “generating permutations using SQL”, you get thousands of hits. It’s an interesting problem if not very useful.
I wrote a solution recently and thought I’d share it. If you’re keen, try tackling it yourself before moving on.

Click through for the script.  It’s an interesting approach and might be worth playing a round of code golf.

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