NULLs in SQL are a mixed blessing. The Three-Valued Logic of SQL has its uses but can cause difficulties to anyone who assumes that NULL has a 'value'. It can make reports go horribly wrong in a number of alarming ways, as Robert Sheldon explains.… Read more
Database programmers are often caught in the paradox of wanting to implement business rules and complex functionality in one place only, but being concerned by the performance hit of having generic functions or procedures that have a wide range of parameters and outputs. Alex Kuznetsov, in an article taken from his book 'Defensive Database Programming with SQL Server', shows how DRY principles can be put in practice with constraints, stored procedures, triggers, UDFs and indexes.… Read more
One of the times that you need things to go right is when you are doing analysis and reporting. This is generally based on time and date. A sure-fire way of getting managers upset is to get the figures horribly wrong by messing up the way that you handle datetime values in SQL Server. In the interests of peace, harmony and a long career in BI, Robert Sheldon outlines some of the worst mistakes you can make when using SQL Server dates.… Read more
Percentiles give meaning to measurements by telling you the percentage of the population being measured who get higher or lower values. They are now easier to calculate in SQL, and are useful for reporting; but are the new analytic functions faster and more efficient than the older methods? Dwain Camps demonstrates, and investigates their relative performance performance.… Read more
SQL Server batch processes are usually run from SQL Agent in background. They can take significant time and resources, especially if they are ETL tasks. Quite often, the responsibility for creating these tasks belongs entirely to the developer. Dennes demonstrates that DBAs can advise and assist with this type of batch job by bringing their expertise to bear on the problem of reducing their impact on the working system to a minimum.… Read more
One of the shocks that a developer can get when starting to program in T-SQL is that there is no simple way of generating documentation for routines, structures and interfaces, in the way that Javadocs or Doxygen provides. To embed the documentation in the source is so obvious and easy that it is a wrench to be without this facility. Phil Factor suggests a solution. … Read more
After answering many forum entries from developers asking for help with dealing with SQL that involved time intervals and ranges, Dwain dreamed of a generalized tool that sets up time slots of various sorts without the need to experiment; that could do the heavy lifting, so that developers could do aggregations and reports based on time intervals without the hard graft. Here is Dwain's dream made reality. … Read more
One of the most certain ways of going wrong with any relational database is to get data conversion wrong. Implicit data conversion is a good feature for the expert but can cause a lot of trouble to the unwary. These boil down to seven ways of failing to get data conversion right. Rob Sheldon explains and gives sage advice on how to avoid the problems… Read more
Refactoring a database object can often cause unexpected behavior in the code that accesses that object. In this article, adapted from his excellent book, Defensive Database Programming with SQL Server, Alex Kuznetsov discusses several techniques that will harden your code, so that it will not break, or behave unpredictably, as a result such changes.… Read more
Spatial Data in SQL Server has special indexing because it has to perform specialised functions. It is able, for example, to break down an indexed space into a grid hierarchy by using a technique called tessellation. This is a rules-based system that, when you compare a shape to an index, works out how many cells in the the grid hierarchy are touched by that shape , and how deep down the grid hierarchy to search. There is powerful magic in Spatial Indexes as Surenda and Roy explain.… Read more
'Temporal' tables contain facts that are valid for a period of time. When they are used for financial information they have to be very well constrained to prevent errors getting in and causing incorrect reporting. This makes them more difficult to maintain. Is it possible to have both the stringent constraints and simple CRUD operations? Well, yes. Dwain Camps patiently explains the whole process.… Read more
Where applications are evolved by gradually molding them to a growing understanding of the business domain, this presents great challenges to database development. If databases are designed too loosely, and initial errors are allowed to fester, the results become harder and harder to refactor until eventually they constitute a database time bomb. Thomas LeBlanc describes how to avoid a few basic, but very common, database time bombs. … Read more
A database must be able to maintain and enforce the business rules and relationships in data in order to maintain the data model. It does this through referential constraints. They aren't complex, but are powerful, especially with the means to attach DRI actions to them. Joe Celko explains all, and pines for the ANSI CREATE ASSERTION statement… Read more
How often do you check your query plans during development to see if they contain any warnings? If you're missing them, it means that you're not getting all those hints about missing indexes, join predicates or statistics. Is the query optimiser trying to tell you about implicit conversions? Dennes shows how to view the warnings in plan cache for a particular database using SQL… Read more
What better way to learn how to construct complex CHECK CONSTRAINTs, use the SQL 2012 window frame capability of the OVER clause and LEAD analytic function, as well as how to pivot rows into columns using a crosstab query? Create the SQL code to score Ten-pin Bowling, of course. Dwain Camps explains the how, what and why.… Read more
Surely, we all know how T-SQL Control-of-flow language works? In fact it is surprisingly easy to get caught out. What, for example, do the BREAK, RETURN and CONTINUE keywords do, precisely, in their various contexts? the answers to this and other questions aren't all entirely obvious, but we're too shy to ask them in public, and risk displaying our ignorance.… Read more
Not all data is discrete; some data types represent a continuum. In SQL, we have to approximate them and live with the special problems of handling continuous data. We need to understand the problems associated with continuous data types, when these will happen, and how it affects constraints and the results of queries. Joe Celko explains.… Read more
Of all the basic SQL operations, the pivot seems to cause the most problems. We can tell from the way that old articles on Simple-Talk on the topic continue to be read. It turns out that there are several questions that come to mind while learning about pivoting, but which are seldom asked on forums. Once more, Robert Sheldon attempts to answer these unspoken questions.… Read more
When you delete a business transaction from the database, there are times when you might want to keep a record of the data for posterity. This can become somewhat complicated if the transaction you need to delete is in a table that is the parent of a deeply nested hierarchy of dependent tables based on the foreign key relationships. In this article, Dwain Camps looks at a tidy means of doing just that.… Read more
Every time you access a relational database to make a query, you have an important decision to make: What is the appropriate isolation level for your query? If you get this wrong, the consequences can be serious. Deadlocks, Dirty reads, Non-repeatable reads, or poor performance. We're in luck, because Robert Sheldon once more answers those questions that are awkward to ask in public.… Read more
