11 December 2010

Pulling the Strings of SQL Server–Part II: Creation and Selection

    It doesn’t make sense to talk about the creation of string data types without talking about their “selection” as people typically want to see also examples at work, and not only learn about theoretical facts. I’m saying that because often programming seems to be like putting together the pieces of a puzzle without knowing how the final image would look like. Anyway, coming back to the topic, what I find great about SQL Server is that it allows to “create” and select a string with a minimum overhead within a simple select statement:

-- selecting a string value 
SELECT 'this is a string' -- string 1 
, 'and this is a second string' --string2 

-- selecting a string value (named columns)  
SELECT 'this is a string' String1  
, 'and this is a second string' String2 
Strings - example1

    Pretty simple, isn’t it? This kind of scripts could be useful especially when debugging logic or testing an expression (e.g. functions at work), the advantage residing in the fact that is not necessary to built a table in order to test simple things. When performing multiple operations with the same values could be handy to store the values in declared variables:  

-- selecting string variables 
DECLARE @string1 varchar(50) 
DECLARE @string2 char(50) 
DECLARE @string3 char(50) = 'this is a string'  

SET @string1 = 'this is a string'  

SELECT @string1 String1 , @string2 String2 
, @string3 String3 
Strings - example2

   As can be seen a not initialized variable has by default the value NULL, fact that could lead to unexpected behavior problems when used in expressions involving multiple variables. Therefore it’s recommended to initialize the values with the empty string or at least to handle the nulls in expressions. Starting with SQL Server 2008 it’s possible to declare and initialize variables within the same statement, so the above code won’t work in previous versions, unless the third declaration is broken into two pieces as per definition of first string.

    It looks like that’s all, at least in what concerns the direct declaration of a string, be it implicit or explicit. However a basic introduction into strings’ creation is incomplete without mentioning the creation of strings from other data types and the various functions that could be used for this purpose. Ignoring the Cast and Convert functions used to explicitly convert the other data types to string data types, there are several other functions for this purpose, namely Char, Space and Replicate.

   Probably a few of you are already familiar with the ASCII (American Standard Code for Information Interchange) character-encoding scheme (vs. binary encoding) used to encode files. ASCII code represents a mapping between number and characters, SQL Server supporting the transformation between two sets through the ASCII and Char functions. If ASCII translates a non-unicode character into an integer, the Char function translates an integer value into a non-unicode character. The integer values range between 0 and 255, they encompassing the 0-9 digits, the characters of English together with the diacritics of other important languages, punctuation signs and several graphical characters. The mapping between the two sets is unique, and as can be seen from the below example based on a common table expression, the functions are inverse:

-- ASCII character values 
AS (    
    SELECT 0 [Index]  
    SELECT [Index]+1 [Index]  
    WHERE [Index]<255 
) SELECT [Index] 
, CHAR([Index]) [Character] 
, ASCII(CHAR([Index])) [ASCII] 

    There is not much to say about the Space and Replicate functions, the Space function returns a string of repeated spaces, while the Replicate function forms a string as a repeated sequence of values. The definition of the Space function could be considered as redundant as long the same output could be obtained by using the space as repeating sequence.

-- Space & Replicate at work 
SELECT Space(1) Example1 
, Space(5) Example2 
, Replicate(' ', 1) Example3 
, Replicate(' ', 5) Example4 
, Replicate('10', 1) Example5 
, Replicate('10', 2) Example6 
, Replicate('10', 3) Example7 

   As per the last statement, the first and third examples, respectively the second and fourth example will return the same values, unfortunately the output of the respective examples it’s not so easy to see. For exemplification, it could have been enriched by comparing or concatenating the strings, though that’s a topic for the next post. 

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