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Category: Syntax

MERGE With Deletion

Published 2017-08-01 by Kevin Feasel

Kevin Wilkie shows an example of deleting data as part of a merge operation:

The last time we were together, we learned how to use the MERGE statement when we wanted to insert rows that didn’t exist and update rows that didn’t. This time we’re going to add onto that. We’re adding the seldom used, but delightfully potent – delete rows that no longer exist in the original table.

MERGE is an enticing but dangerous piece of syntax.  It looks so nice until you realize how many bugs and oddities there are in the command.

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EXISTS Is Self-Contained

Published 2017-06-30 by Kevin Feasel

Shane O’Neill ponders an existential problem:

So, drinking my first (of many) coffee of the day, I asked him what was wrong with it.

I have two tables. 1 with values 1,2,3 & the other with values 1,2,3,4,5. When I use delete exists, it should just delete 1,2,3 but table1 is always empty.

Hmmm, not an unreasonable assumption I suppose so I asked him for his code.

Read on for Shane’s explanation, though he doesn’t like the verbosity.  My version is, what happens in EXISTS stays in EXISTS.  It just returns a signal to the outer query saying yea or nay and the outer query does its thing accordingly.  In this case, if you want to tie results back to the delete operation, use IN (the ANSI standard way) or JOIN (typically my preferred way, given that IN can get dicey with more complex criteria).

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New T-SQL Functions

Published 2017-06-28 by Kevin Feasel

Dennes Torres shows off four T-SQL functions releasing in SQL Server 2017:

Translate

Translate does the work of several replace functions, simplifying some queries.

The function is called ‘Translate’ because its main objective: transform one kind of information in another by doing a bunch of replaces.

For example: GeoJson and WKT are two different formats for coordinates. In GeoJson a coordinate is represented using the format ‘[137.4, 72.3]’ while in WKT a point is represented using the format ‘(137.4 72.3)’.

We would need several ‘Replace’s to transform GeoJson format in WKT format and the reverse. The ‘Translate’ function can do this easily.

I knew the first three, but Translate sneaked right past me.

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Ditching IsNumeric()

Published 2017-06-26 by Kevin Feasel

Phil Factor on the IsNumeric() function:

IsNumeric() is actually answering the question ‘Can this string be converted or ‘cast’ to a numeric datatype by SQL Server?’ It is pretty good at getting this answer right but it doesn’t tell you which datatype. It also isn’t interested in such things as overflow.

This problem of IsNumeric() was solved in SQL Server 2012. There was no way of morphing IsNumeric() intro something more valuable, so Try_Cast(), Try_Parse(), and Try_Convert() were introduced. The introduction of these system functions really solve the problem, unless you are still on earlier versions than SQL Server 2012. I’ll show how to do the same thing in previous versions of SQL Server, and demonstrates one or two tricks you can do with these functions.

I’ll stick with the post-2012 version, please.

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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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Using Common Table Expressions To Drive Queries

Published 2017-06-01 by Kevin Feasel

Lukas Eder wants one result set which returns records using predicate B if and only if there were no records using predicate A:

We’ve seen that we can easily solve the original problem with SQL only: Select some data from a table using predicate A, and if we don’t find any data for predicate A, then try finding data using predicate B from the same table.

Oracle and PostgreSQL can both optimise away the unnecessary query 2 by inserting a “probe” in their execution plans that knows whether the query 2 needs to be executed or not. In Oracle, we’ve even seen a situation where the combined query outperforms two individual queries. SQL Server 2014 surprisingly does not have such an optimisation.

Interesting totally-not-a-comparison between the three database products.  There are some things I’d ideally like the SQL Server optimizer to do with common table expressions, but as Lukas notes, it doesn’t, so user beware.

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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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Built-In SQL Server Functions

Published 2017-05-26 by Kevin Feasel

Tywan Terrell has a listing of various functions built into SQL Server:

SQL Server starting with 2012 ship with a robust set of functions that can be used to make code perform faster, with fewer lines of code. The functions  can be used in ETL Process to provide better error handling. A example of this would be the Try_Parse function that allows you to check if a value can be converted.

Another example would be using the FIRST_VALUE() and LAST_VALUE() functions which work against a set of data. These functions are very useful when looking for things like month over month averages and when doing calculations. The table below contain a list of function that are supported starting with 2012 along with some examples of how to use them.

He breaks them down into four categories and provides examples.  Functions can bring their own set of problems with query performance, but most of them can be very useful.

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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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