Britain has “really everything in common with America nowadays, except, of course, language.” Said Oscar Wilde, in the Centerville Ghost (1887) whilst George Bernard Shaw is quoted as saying that the “The United States and Great Britain are two countries separated by a common language.”
There are similarities and differences between SQL and R, which might be confusing. However, I think it can be illuminating to understand these similarities and differences since it tells you something about each language. I got this idea from one of the attendees at PASS Summit 2015 and my kudos and thanks go to her. I’m sorry I didn’t get her name, but if you see this you will know who you are, so please feel free to leave a comment so that I can give you a proper shout out.
If you are looking for an intro to R from the Excel perspective, see this brilliant blog here. Here’s a list onto get us started. If you can think of any more, please give me a shout and I will update it. It’s just an overview and it’s to help the novice get started on a path of self-guided research into both of these fascinating topics.
|R||SQL / BI background|
|A Factor has special properties; it can represent a categorical variable, which are used in linear regression, ANOVA etc. It can also be used for grouping.||A Dimension is a way of describing categorical variables. We see this in the Microsoft Business Intelligence stack.|
|in R, dim means that we can give a chunk of data dimensions, or, in other words, give it a size. You could use dim to turn a list into a matrix, for example||Following Kimball methodology, we tend to prefix tables as dim if they are dimension tables. Here, we mean ‘dimensions’ in the Kimball sense, where a ‘dimension’ is a way of describing data. If you take a report title, such as Sales by geography, then ‘geography’ would be your dimension.|
|R memory management can be confusing. Read Matthew Keller’s excellent post here. If you use R to look at large data sets, you’ll need to know
– how much memory an object is taking;
– 32-bit R vs 64-bit R;
– packages designed to store objects on disk, not RAM;
– gc() for memory garbage collection
– reduce memory fragmentation.
|SQL Server 2016 CTP3 brings native In-database support for the open source R language. You can call both R, RevoScaleR functions and scripts directly from within a SQL query. This circumvents the R memory issue because SQL Server benefits the user, by introducing multi-threaded and multi-core in-DB computations|
|Data frame is a way of storing data in tables. It is a tightly coupled collections of variables arranged in rows and columns. It is a fundamental data structure in R.||In SQL SSRS, we would call this a data set. In T-SQL, it’s just a table. The data is formatted into rows and columns, with mixed data types.|
|All columns in a matrix must have the same mode(numeric, character, and so on) and the same length.||A matrix in SSRS is a way of displaying, grouping and summarizing data. It acts like a pivot table in Excel.|
|<tablename>$<columnname> is one way you can call a table with specific reference to a column name.||<tablename>.<columname> is how we do it in SQL, or you could just call the column name on its own.|
|To print something, type in the variable name at the command prompt. Note, you can only print items one at a time, so use cat to combine multiple items to print out. Alternatively, use the print function. One magic feature of R is that it knows magically how to format any R value for printing e.g.
|PRINT returns a user-defined message to the client. See the BOL entry here. https://msdn.microsoft.com/en-us/library/ms176047.aspx
CONCAT returns a string that is the result of concatenating two or more string values. https://msdn.microsoft.com/en-GB/library/hh231515.aspx
|Variables allow you to store data temporarily during the execution of code. If you define it at the command prompt, the variable is contained in your workspace. It is held in memory, but it can be saved to disk. In R, variables are dynamically typed so you can chop and change the type as you see fit.||Variables are declared in the body of a batch or procedure with the DECLARE statement and are assigned values by using either a SET or SELECT statement. Variables are not dynamically typed, unlike R. For in-depth look at variables, see Itzik Ben-Gan’s article here.|
|ls allows you to list the variables and functions in your workspace. you can use ls.str to list out some additional information about each variable.||SQL Server has tables, not arrays. It works differently, and you can find a great explanation over at Erland Sommarskog’s blog. For SQL Server 2016 specific information, please visit the Microsoft site.|
|A Vector is a key data structure in R, which has tons of flexibility and extras. Vectors can’t have a mix of data types, and they are created using the c(…) operator. If it is a vector of vectors, R makes them into a single vector.||Batch-mode execution is sometimes known as vector-based or vectorized execution. It is a query processing method in which queries process multiple rows together. A popular item in SQL Server 2016 is Columnstore Indexes, which uses batch-mode execution. To dig into more detail, I’d recommend Niko Neugebauer’s excellent blog series here, or the Microsoft summary.|
There will be plenty of other examples, but I hope that helps for now.