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9.4.2 String Collating Support »
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9.4.1. The Character Definition Arrays

Each simple character set has a configuration file located in the sql/share/charsets directory. The file is named MYSET.xml. It uses <map> array elements to list character set properties. <map> elements appear within these elements:

  • <ctype> defines attributes for each character

  • <lower> and <upper> list the lowercase and uppercase characters

  • <unicode> maps 8-bit character values to Unicode values

  • <collation> elements indicate character ordering for comparisons and sorts, one element per collation (binary collations need no <map> element because the character codes themselves provide the ordering)

For a complex character set as implemented in a ctype-MYSET.c file in the strings directory, there are corresponding arrays: ctype_MYSET[], to_lower_MYSET[], and so forth. Not every complex character set has all of the arrays. See the existing ctype-*.c files for examples. See the CHARSET_INFO.txt file in the strings directory for additional information.

The ctype array is indexed by character value + 1 and has 257 elements. This is an old legacy convention for handling EOF. The other arrays are indexed by character value and have 256 elements.

ctype array elements are bit values. Each element describes the attributes of a single character in the character set. Each attribute is associated with a bitmask, as defined in include/m_ctype.h: 

#define _MY_U   01      /* Upper case */
#define _MY_L   02      /* Lower case */
#define _MY_NMR 04      /* Numeral (digit) */
#define _MY_SPC 010     /* Spacing character */
#define _MY_PNT 020     /* Punctuation */
#define _MY_CTR 040     /* Control character */
#define _MY_B   0100    /* Blank */
#define _MY_X   0200    /* heXadecimal digit */

The ctype value for a given character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_MY_U) as well as a hexadecimal digit (_MY_X), so its ctype value should be defined like this: 

ctype['A'+1] = _MY_U | _MY_X = 01 | 0200 = 0201

The bitmask values in m_ctype.h are octal values, but the elements of the ctype array in MYSET.xml should be written as hexadecimal values.

The lower and upper arrays hold the lowercase and uppercase characters corresponding to each member of the character set. For example: 

lower['A'] should contain 'a'
upper['a'] should contain 'A'

Each collation array is a map indicating how characters should be ordered for comparison and sorting purposes. MySQL sorts characters based on the values of this information. In some cases, this is the same as the upper array, which means that sorting is case-insensitive. For more complicated sorting rules (for complex character sets), see the discussion of string collating in Section 9.4.2, “String Collating Support”.