Category: T-SQL

Allan Hirt prefers ANSI-89 join syntax:

Pedro Lopes who is now on the SQL Server dev team wrote the blog post “T-SQL Misconceptions – JOIN ON vs. WHERE” which does a good job of explaining the “it depends” around the newer style of joins. Why link this article? Microsoft deprecated the OUTER JOIN operators in SQL Server 2008. Two other sources of information on this are here and here (the first is Ward Pond’s old technet blog, and sadly will probably go away soon). If you’re keeping score at home, WHERE clauses are not deprecated except if you’re using *= and =*). The changes people made were wholly unnecessary and as the author, the newer stuff is harder to decipher than what I originally did. They were putting their own biases onto things.

I personally do not like ANSI-89 syntax because it blurs the lines between filters and join criteria. Those are separate things serving different purposes and keeping them separate clarifies queries more than it obscures. Allan’s example doesn’t have any filters but in a more complex scenario with several filters and several join criteria, it can require extra care figuring out what’s going on, especially with multi-column join criteria and filters mixed in (meaning it’s not join criteria and then filters, but a mishmash of the two).

Dave Bland shows how you can build a dynamic pivot to see performance counter data over a stretch of time:

The next step is to write the code to capture the counter values and insert the data it the temporary table created above.  Because we need to capture the values over a period of time, the WAITFOR DELAY is used.  In this case the delay is 10 seconds, although you can change this to suit your needs. Of course, don’t forget to increment the counter variable. You will need to determine what counters you would like to capture.  Notice in the WHERE clause, we are looking for an instance_name of ” or ‘_total’.  This will allow the code to only capture one row for each counter.  The number 10 is the number of times we want to capture the counter values.  If you want to capture the data more frequently, simpley modify the number of seconds in the WAITFOR DELAY.  Here is link to my post in this topic, WAITFOR.

Dynamic pivoting in SQL is unnecessarily difficult, especially compared to languages like R.

Jeffry Schwartz compares the performance of STRING_AGG in SQL Server 2017 versus the tried and true method of FOR XML PATH for string concatenation:

Two different T-SQL methods were used for testing.  The FOR XML PATH solution incorporates the reverse and stuff functions along with FOR XML PATH.  The String_agg solution is much simpler, but is only available on SQL Server 2017 currently.

The FOR XML PATH method has been documented thoroughly over the years in many articles, so these explanations will not be repeated here.  However, two points are worth mentioning.  The first one is that the @strSeparator (in this case ‘ or  ’) string will reside at the end of the constructed string (in this case @strFilter).  Since the last occurrence of @strSeparator must be removed to prevent a syntax error, characters must be dropped from the end of the string.  Although it may seem obvious that four characters (the length of @strSeparator) should be removed, actually only three need be removed because the end of the constructed string is “r” not “ “.  Therefore, the length argument of the left function specification is the length of @strFilter minus three.

I like STRING_AGG a lot, primarily because I can remember it and I can explain it to people relatively new to SQL Server. Read on to see how they both compare in terms of performance.