By Rob Gravelle
My last DatabaseJournal article, All About the Crosstab Query, described how to formulate an SQL statement for generating a cross tabulation query. My original intention for today's follow up was to explore the use of stored procedures to make crosstab generation more dynamic. That was until a bug sent me on a search for answers. What I found was intriguing enough to make me set aside my original topic, so that I could now relate what I discovered about the handling of import data encoding. I think that you'll agree that it's a journey well worth taking!
Character Encoding and Collation Described
A character encoding is a way of mapping a character (the letter 'A') to an integer in a character set (the number 65 in the US-ASCII character set). With a limited character set, such as US-ASCII, which includes the twenty-six letters of the English alphabet, both lowercase and uppercase, numbers from 0 to 9, and some punctuation, fitting this into a single byte is not a problem. But when dealing with other languages like German, Swedish, Hungarian, and Japanese, you start to hit the boundaries of the 8-bit byte. This can happen when you try to create a character set to represent two languages, or even a single language like Japanese.
In an effort to account for the profusion of languages and scripts in the modern world, a number of different character encodings have been ascribed for mapping different characters to integers. For character sets that wouldn't fit in a single byte, double-byte character sets were created, along with multi-byte character sets that use a special character to signal a shift between single-byte and double-byte encoding.
The Unicode Consortium came together to create a specification for a character encoding that would be able to encompass the characters in all written languages. The result was the Unicode character set. The two most common are UCS-2, which encodes everything as two-byte characters, and UTF-8, which uses a multi-byte encoding scheme that extends US-ASCII.
ISO-8859-1 is the most common character set used for Western languages, and it is extended by the Windows-1252 character set to include some other characters, such as the Euro (€) and trademark symbol (™). Because Windows-1252 is a superset of ISO-8859-1, the character set is known as latin1 to MySQL. (It does not recognize ISO-8859-1 as being a distinct character set.)
So Why Can't We Just Use UCS-2 or UTF-8 for Everything?
The main reason that it isn't practical to always use Unicode is that it wastes bandwidth when using only a single language. In TIS-620, the single byte code page for Thai, all characters takes up one byte, whereas in UTF-8, Thai characters take up three bytes each. Many people think UTF-8 is efficient because ASCII characters take up only one byte, but in reality, UTF-8 can be highly inefficient when most of your file consists of characters outside of the ASCII set. For example, if half your file consists of ASCII, and the other half is Thai, then saving the file in UTF-8 makes it take up twice as much space than TIS-620 would.
In situations where storage space is at a premium, and ASCII plus one script is used, using an older one byte character set can make a lot of sense. Unicode is necessary when using multiple scripts in one file, and two or more of the scripts use different code pages (e.g.: you can mix Thai and English, because TIS-620 also includes the ASCII symbols, but you cannot mix Thai and Greek without using Unicode, because Thai and Greek require different code pages).
A collation comprises the rules governing the proper use of characters for either a language, such as Greek or Polish, or an alphabet, such as Latin1_General. The collation attribute is used by MySQL for the sorting of characters in relation to one another, and not for encoding specifically.
Each SQL Server collation specifies two properties:
* The sort order to use for nchar, nvarchar, and ntext Unicode data types as well as for non-Unicode character data types (char, varchar, and text). A sort order defines the sequence in which characters are evaluated in comparison operations.
* The code page used to store non-Unicode character data.
If you'd like to read up more on collation, there's an informative DatabaseJounal article on collation by Muthusamy Anantha Kumar (aka The MAK).
A Tale of Two Character Sets
MySQL ships with a Latin-1 as the default encoding - actually latin1_swedish_ci – presumably because it is used by the majority of MySQL customers. In MySQL 4.1.12 or greater, data is imported in UTF8 encoding. While this does allow the maximum number of characters codes, it can still present problems when the incoming data is in a different character encoding. What's worse is that you may not know about the problem until a lot of work is required to undo the damage!
I discovered this first-hand when I imported data from an Access database to test my crosstab SQL code in MySQL. The transfer only ran into one snag, when the date formats were inconsistent. I fixed that problem by formatting the dates in the universal "yyyy-mm-dd" format. Here is the Access query used to extract the data:
Format([CREATION_DATE],"yyyy-mm-dd") AS [CREATION DATE],
To my surprise, running the query on the imported data produced the following bizarre results, as seen in this screenshot of my HeidiSQL Windows client:
It seemed that the MONTHNAME function was misbehaving and returning a HEX number instead of a string. Upon further experimentation, I concluded that any function on the dates were returning HEX values. A few Internet searches later, I came to understand that this sort of thing was quite common as I read account after account of people having to jump through hoops to translate character codes to the correct number.
In my case, the discrepancy between the two encodings was caused by Access when I saved the data to a .csv (comma-separated values) file. MS Access uses the "Windows-1252" character encoding when exporting to text. You can test this by exporting data as an HTML page. In it, there will be a META tag that declares the character encoding for the page:
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=Windows-1252">
The simplest solution that I found was to use the MySQL CONVERT function in the query. It accepts a value and translates it to the encoding format that you specify following the USING keyword. Here is how I used the CONVERT function in my SQL code to fix the encoding problem:
Mysql>SELECT CONVERT(MONTHNAME(CREATION_DATE) USING latin1) AS 'Month',
The solution that I employed only affected the output of the query. Other solutions can be utilized at import time or applied to the entire server, database, table, or field level. We'll be looking at these after we get to our previously scheduled crosstab stored proc, in the next article.