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Category: Syntax

Finding Out Whodunnit Using The Transaction Log

Published 2017-09-05 by Kevin Feasel

David Fowler shows us how to figure out which user made a bad data change when you don’t have auditing mechanisms in place:

So it’s looking like things are in a bad way, obviously we could go to a backup and get the old values back but that’s never going to tell us who made the change.  So that transaction log again, how do we actually go about getting our hands dirty and having a look at it.

Well there’s a nice little undocumented function called fn_dblog.  Let try giving that a go and see what we get back. By the way, the two parameters are the first and last LSNs that you want to look between.  Leaving them as NULL with return the entire log.

This is great unless you have connection pooling and the problem happened through an application.  In that case, the returned username will be the application’s username.

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Substrings In SQL Server Versus Oracle

Published 2017-08-25 by Kevin Feasel

Daniel Janik continues his SQL Server versus Oracle syntax comparison series:

Parsing strings is a feature that is often needed in the database world and SUBSTRING/SUBSTR are designed to do just that. I find it interesting how these two platforms approached the functions differently and that’s definitely shows how you can do many things to get to the same answer.

It’s a short post, but Daniel does show one big difference between the Oracle and SQL Server substring functions.

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Palindromes In SQL

Published 2017-08-22 by Kevin Feasel

Lukas Eder rises to a challenge:

SQL is a really cool language. I can write really complex business logic with this logic programming language. I was again thrilled about SQL recently, at a customer site:

But whenever I tweet something like the above, the inevitable happened. I was nerd sniped. Oleg Šelajev from ZeroTurnaround challenged me to prove that SQL is so awesome:

Given a string, find all substrings from that string, which are palindromes. Challenge accepted! (For the moment, let’s forget about algorithmic complexity.)

His answer is in Postgres syntax, and a commenter includes Oracle syntax.  T-SQL is left as an exercise for the reader.

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Your Reminder Not To MERGE

Published 2017-08-18 by Kevin Feasel

Kevin Wilkie points out the numerous problems with the MERGE operator:

Now, when I last posted, I’m sure you thought I was done talking about the MERGE statement. You are so wrong, compadre! One more post is absolutely needed!

There are a few issues with the MERGE statement. Well, as of this writing, there are 361 possible issues according to Microsoft Connect – the actual website where Microsoft checks to see what issues exist!

So, if you want to use the MERGE statement, please read through every issue listed on the link above and make sure that none of those scenarios could exist for you. If they don’t, great. Knock yourself out and use it.

But wait, there’s more!  Read on to see what else could be a problem.

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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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Substrings: Powershell Versus T-SQL

Published 2017-08-14 by Kevin Feasel

Shane O’Neill contrasts the SUBSTRING function in T-SQL with Powershell’s Substring method:

The main difference that I can see when using SUBSTRING() in SQL Server versus in PowerShell is that SQL Server is very forgiving.

If you have a string that is 20 characters longs and you ask for everything from the 5th character to the 100th character, SQL Server is going to look at this, see that the string does not go to the 100th character, and just give you everything that it can.

It’s a small difference but an important one.

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Date Conversions In Oracle And SQL Server

Published 2017-08-14 by Kevin Feasel

Daniel Janik compares Oracle and SQL Server date conversion functions:

There are many ways to create a date from a string. First you’ve got the CONVERT() function and the CAST() function. Next you’ve got DATEFROMPARTS(), DATETIMEFROMPARTS(), DATETIME2FROMPARTS(), SMALLDATETIMEFROMPARTS(), TIMEFROMPARTS(), and DATETIMEOFFSETFROMPARTS().

That’s a lot of functions for one simple task isn’t it? To be fair, it’s really more than 1 simple task. Each of these functions is meant to be paired with the matching data type so you get just what you want. To go along with these you also have the ISDATE() function which tests the value to be sure it’s a date.

I never liked the verbosity of the Oracle TO_DATE() function…but I am biased.

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Pagination In Oracle Versus SQL Server

Published 2017-08-11 by Kevin Feasel

Daniel Janik is currently running an Oracle versus SQL Server series, looking at how the two database systems expose common functionality.  His latest topic is pagination:

Today’s topic is Pagination. Paging is a really important feature for web pages and applications. Without it you’d be passing large amounts of data to the application and expecting the application code to figure out which rows it needed to display.

Thankfully, someone smart came up with a way to do this on the database so you’re not returning gigs and gigs of data to the webserver to sort through.

Read on to see how the two platforms do this.

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Working With UTC And Local Times

Published 2017-08-09 by Kevin Feasel

Jo Douglass shows how to use the DATETIMEOFFSET data type and AT TIME ZONE syntax to convert between UTC and local times:

Run select SysDateTimeOffset(); and you should see a date and time which mirrors your server’s current time, plus a time zone offset showing its current offset from UTC; this includes any time zone offset, plus any daylight savings time offset.

If I were to run this (from the UK) on August 15th, 2017 while my clock is showing that it’s noon exactly, I would get 2017-08-15 12:00:00.0000000 +01:00; the +01:00 offset is because the UK is offset by one hour from UTC during daylight savings. The datetime2 portion of a datetimeoffset is in local time, not UTC.

My normal operation is to store everything in UTC and let the application convert to local times.  That allows you to compare dates much more easily and reduces confusion around daylight savings time.

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Left Versus Right Joins

Published 2017-08-07 by Kevin Feasel

Denis Gobo doesn’t like RIGHT JOIN:

Do you use RIGHT JOINs? I myself rarely use a RIGHT JOIN, I think in the last 17 years or so I have only used a RIGHT JOIN once or twice. I think that RIGHT JOINs confuse people who are new to databases, everything that you can do with a RIGHT JOIN, you can also do with a LEFT JOIN, you just have to flip the query around

So why did I use a RIGHT JOIN then?

Don’t be lazy; switch out those right joins.  The trick is that for every RIGHT JOIN statement, there is an equivalent statement which does not use RIGHT JOIN.  The percentage of the time that you might benefit from RIGHT JOIN is so low that the fixed costs of mentally processing what’s going on tend to overwhelm the slight benefit of that style of join.

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