Let’s start by defining a subset and why you would require a data subset? When dealing with the development, testing and releasing of new versions of an existing production database, developers like to use their existing production data. In doing so, the development team will be hit with the difficulties of managing and accommodating the … Read more
In this article, I will discuss the history and thinking behind several types of logic that are typically associated with writing relational database code. Because of how relational databases implement NULLs, it definitely can make some processes more complex than expected by may programmers (inexperienced and experienced alike!) Three Valued Logic Missing or UNKNOWN values … Read more
Ever wonder all the reasons that we use databases instead of file systems? While we don’t think of it too much anymore, the first reason that databases came into existence was to remove redundancies. The first source of redundancy back in the dark ages, when I was just beginning the program, was a product called … Read more
Packing intervals is a classic SQL task that involves packing groups of intersecting intervals to their respective continuous intervals. In mathematics, an interval is the subset of all values of a given type, e.g., integer numbers, between some low value and some high value. In databases, intervals can manifest as date and time intervals representing … Read more
Aggregation is a widely used way to summarize the content of a database. It is usually expressed with GROUP BY clause or just using aggregate functions (like COUNT or SUM). When the database engine executes a query with aggregations, it produces individual rows need to compute the required output and then performs the aggregation as … Read more
Imagine this: you have several directories full of SQL script files, and you need to know where a certain table is used. You’d rather like the context too, so you can check the whole SQL Expression and work out why it is running so slowly. Maybe, from that same daunting set of several directories, you … Read more
Tell me if you’ve heard this one before: I changed data from my application, but when I checked the database, I couldn’t see the change! I’ve seen this. Loads. It can be quite perplexing for folks because they expect to see an error message if the insert, update, or delete failed. I put this post … Read more
The SQL NTILE() is a window function that allows you to break a table into a specified number of approximately equal groups, or <bucket count>. For each row in a grouping, the NTILE() function assigns a bucket number representing the group to which the row belong starting at one. The syntax of the NTILE() function … Read more
Dr. Codd first described the relational model in a paper in Communications of the ACM (CACM 13 No 6; June 1970). Some more work followed up after that by other people, giving us normal forms and other things we have taken for granted for 50+ years. I was looking at some postings on a SQL … Read more
When people start learning a new field, for example T-SQL, it’s tempting to spend very little time trying to understand the fundamentals of the field so that you can quickly get to the advanced parts. You might think that you already understand what certain ideas, concepts and terms mean, so you don’t necessarily see the … Read more
When you’re learning SQL DML, the most complicated clause is typically the GROUP BY. It’s a fairly simple grouping based on values from the FROM clause in a SELECT statement. It’s what a mathematician would call an equivalence relation. This means that each grouping, or equivalence class, has the same value for a given characteristic … Read more
In this article, Joe Celko gives us a history of the different character sets that are used in computing and how that can pertain to your usage in relational databases. Some of these you may have never heard of!… Read more
The VALUES clause makes inserting literal values into a table simple and easy. In this article, Joe Celko explains how to use the VALUES clause.… Read more
SQL Server provides two ways to include an incrementing number in a table. Greg Larsen explains how to replace an identity column with a sequence number.… Read more
Database systems have powerful features built right in that can ensure the integrity of data. In this article, Joe Celko discusses keys and constraints.… Read more
Dr. Codd proposed the normalization rules we used to design databases, but did you know that he also came up with rules that vendors must meet to call their products relational database systems? In this article, Joe Celko explains the thirteen RDBMS rules.… Read more
Joe Celko explains why questions requiring yes or no answers are more complicated than you might realize in both spoken language and computing.… Read more
