20 June 2010

Troubleshooting – Part II: Troubleshooting query issues: Errors

    Some weeks ago I wrote a post on troubleshooting, looking at the topic from problem solving perspective. In what concerns the troubleshooting of SQL scripts, now it depends also on the type of DML statement used or on the type of issues the developer deals with – errors thrown by the database engine, wrong number of records returned by a query, performance, accessibility or reusability related issues.

    Many of the errors come normally from wrong usage of syntax and functionality or from misspells, in many situations the database query engine providing enough information, leading the developer closer to issue’s resolution, at least by narrowing the range of search, and here the problem solving approach could be useful. In such situations it is useful to do a quick search in the local/online documentation or resources using the error number or the full/partial error message. I know that’s not rocket science, though I’ve seen people acting like hypnotized by the error message, not knowing what do to next. A quick search using a powerful search engine like Google.com, Bing.com or Yahoo.com could lead you to the solution or help gathering more data about the possible causes. More likely that there was somebody in the same situation and has posted a question or even the answer somewhere in a forum or a blog. It’s always a good idea and time-effective to do a little research before posting a new question in a forum. Especially between beginners there are people who don’t know to make use of the documentation, help tools or even a search engine, therefore it could be useful for them to take some time and look on some tips on how to make better use of such tools.

    In general when an error is thrown is provided an error number that has a specific meaning for the vendor, a general description, and in fortunate cases also the line where it occurs. For example in SQL Server the execution of the below query based on AdventureWorks database will return the next error message:

dbo.Purchasing.PurchaseOrderHeader POH

Msg 208, Level 16, State 1, Line 1
Invalid object name 'dbo.Purchasing.PurchaseOrderHeader'

        Even if in this case the error was created deliberately by using the name of a schema (e.g. dbo) instead of database’s name, this doesn’t mean that the cause/solution is so simple. For example the same error message could be thrown by an issue related to KB837970, KB310882, KB289551 and several other articles that could be found by using the following search text in Google:
site:support.microsoft.com Microsoft.com "Msg 208, Level 16"

    Unfortunately in what concerns the errors appearing in queries, the localization of errors is not so clean, depending also on the order in which the query is parsed by the engine, thus the issue might be shown as occurring at a line other than its actual location, namely at the line where the query parsing finds an issue.
    When the error message is in other languages than English, it makes sense to try to find the corresponding error message in English and use it in search because, as a quit important volume of documentation and other type of information are in English, the chances to find something are higher. Normally that could be done by using the error code, which in theory should have the same meaning independently of the language used.

    Sometimes, when you didn’t knew how to do something and just used your inspiration and tried to use some functionality, and you are not fully aware of its implications and constraints, then it makes sense to consult the documentation or do a search on the web on how to do a certain task (e.g. How to load data from Oracle to SQL Server). 10 minutes spent in research could help you save a few hours later, so don’t jump blindly into coding! In many situations it could happen that a solution is already available, and if you find already the code, please consider not using it blindly, try to understand the how and why, its limitations and how it could be eventually improved, and some sorrow testing might be necessary too. Don’t try to reinvent the wheel just for the sake of it, in what concerns programming there are more likely others who had the same problem and found a neat solution to it. This doesn’t mean that you’ll have to do that for every problem you’ll have, but for tasks having higher complexity than the tasks you are accustomed with, the need for that decreasing with the level of expertise. Doing some research on a given topic can be time consuming, but could prove to be useful especially when other developers used other technique. Actually once you understood the idea, you could attempt to reconstruct the logic with your own coding and styling conventions, of course, not always it makes sense to do that.

    You’ll have to consider also copyright infringements, but in theory the developers who posted code on the web for further reuse, want only some minimal credit for that (e.g. posting a link to the website where the code is found, mentioning developers’ name, etc.), while in some situations you could reuse the code only if you add to it substantial changes. Both statements apply not only to code but also other type of work (content) available freely on the web (e.g. articles, tutorials, etc.).

    In case the search was unsuccessful then it makes sense to post your question on a forum, specifically in the area closest to your problem. Posting a question in the wrong forum has quite high chances to lead nowhere! Some forum members are quite critical about the users who post questions without doing some research, especially when there is already an answer available on the same forum. On forums, users have to be aware that they are not only wasting their time, but also readers’ time. On the other side, the more the points where the same question is posed, the higher are the chances for others to reach to them, and eventually get an answer, this depending also on how search engines index the web pages. 
    When the same error appears for the second time, in theory the developer should know already how to solve it or the possible cause(s). Because the solutions are not always so simple to remember as is in human nature to forget, it makes sense to document the issues by describing the problem and the solutions (preferably with code) to such a level that it could understood also by other professionals.

    When the number of errors occurring in your SQL scripts is quite high, especially when the same type of errors occurs, then maybe it makes sense to take a step back and ask yourself why that happens. Sometimes it just means that is time to take a break, though, more obvious, is maybe needed to review the theory/documentation, make use of defensive programming techniques or use an editor with powerful IntelliSense functionality that checks the syntax and highlights the errors as you type. If you are a beginner, don’t worry, with time coding and troubleshooting becomes much easier.

    All the above ideas are simple common knowledge between developers, though such knowledge comes from (prolonged) experience – a combination of learning by doing and solving issues – “the school of hard knocks”. I just hope the above lines and the coming posts will be helpful to you.

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