#Mysql database server express stackoverflow code#So now, skipping the intermediate explanatory step as it is quite similar to what we did before, we can arrive at the following code to calculate islands from gaps: Finally, we’ll un-pivot and add a row number that groups each resulting sequence number in pairs (note that you could either add 1 to or subtract 1 from the row number before dividing by 2). We need to add a row, and that row will be the first and last sequence number in our series. Let’s consider a data transformation as follows:įor this solution, we’ll take the end point and add one, while we subtract one from the start point. If it is possible to convert Islands to Gaps, perhaps it is also possible to convert Gaps to Islands. In a moment we’ll see if these results can be reproduced and how they scale. We’ll also move the fastest islands bit of code magic into a Common Table Expression (CTE) so we can focus on our gaps solution.Įlapsed times for the new CAV (Cross Apply Values) methods appear quite competitive, and both logical IOs and CPU time appear improved. We’ll choose the latter because as this article: An Alternative (Better?) Method to UNPIVOT seems to demonstrate, that approach may be the swiftest. There are at least three methods to un-pivot two columns: 1) using the SQL UNPIVOT keyword, 2) doing a UNION ALL between each of the columns and 3) using the CROSS APPLY VALUES (CAV) approach to UNPIVOT. This suggests to us that UNPIVOTing the islands might give us something to work with to arrive at the gaps. Each pair of colored, circled numbers in the rightmost table perfectly represents our gaps. If we first insert two columns and fill the first with one less than the StartSeqNo and the second with one greater than the EndSeqNo, we can circle the numbers that represent the gaps. Let’s take a look at the islands results to see how it might be converted into gaps. Since we have relatively a fast method of determining islands, perhaps it’s possible to convert that solution into one that will identify our gaps. Let’s dip our toe in the pond by looking at some sample data. While doing so, I happened upon a rather different solution to the problem, and was sufficiently intrigued to develop a test harness to evaluate the way that the various algorithms performed and scaled. My interest was piqued by that article and I played about with the examples to try to gain a little better insight. Chapter 5 of SQL Server MVP Deep Dives, Gaps and Islands by Itzik Ben-Gan, is probably the best and most thorough explanation of the main solutions. It is complex and interesting task to come up with a solution that performs and scales well. So if you want to represent the event “consignment arrived at the depot” and list the package numbers that did arrive, you’d want to group them like 1-10, 15-18, etc. If packages are missing those represent the gaps. Typically you scan all packages when a consignment reaches the depot. A typical example might occur in the express distribution business where a consignment has many packages numbered sequentially. The ‘Gaps and Islands’ problem isn’t entirely an academic game, since a number of business processes demand some way of detecting gaps and islands in sequences. In all cases, the sequences do not contain duplicates. Islands and gaps appear in all sorts of sequences, be they IDENTITY columns where some rows have been removed or dates that occur in sequence (but some are missing). The ‘Gaps and Islands’ problem is that of using SQL to rapidly detect the unbroken sequences, and the extent of the gaps between them in a column. Islands are unbroken sequences delimited by gaps. The word ‘Gaps’ in the title refers to gaps in sequences of values. MySQL has replication and high availability if you go with MySQL Cluster.Gaps and Islands in SQL Server data - Simple Talk Skip to content I would recommend going with Linux for MySQL as it is usually a more stable platform and gives you more flexibility since many tools for MySQL are written for Linux. MySQL provides InnoDB engine which is fully ACID compliant, you can have online backups by using mysqldump with correct options with InnoDB tables or by using Xtrabackup from Percona. If you do not want to face all those restrictions you should go with MySQL. If you go to and click on SQL Server Express, you will see just how restrictive the limitations are:Įxpress will run only using one CPU, will only use up to 1 GB of RAM, and will allow you to have a database up to 10 GB only. Take it with a grain of salt, as the information is provided by Microsoft, the maker of SQL Server. Just go to and pick MySQL in the dropdown box. Microsoft provides a comparison between MySQL, SQL Server Express, and SQL Server Enterprise.
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