Rechercher dans le manuel MySQL

12.5.3 Character Set and Collation of Function Results

Contents

MySQL has many operators and functions that return a string. This section answers the question: What is the character set and collation of such a string?

For simple functions that take string input and return a string result as output, the output's character set and collation are the same as those of the principal input value. For example, UPPER(X) returns a string with the same character string and collation as X. The same applies for INSTR(), LCASE(), LOWER(), LTRIM(), MID(), REPEAT(), REPLACE(), REVERSE(), RIGHT(), RPAD(), RTRIM(), SOUNDEX(), SUBSTRING(), TRIM(), UCASE(), and UPPER().

Note

The REPLACE() function, unlike all other functions, always ignores the collation of the string input and performs a case-sensitive comparison.

If a string input or function result is a binary string, the string has the binary character set and collation. This can be checked by using the CHARSET() and COLLATION() functions, both of which return binary for a binary string argument:

  1. mysql> SELECT CHARSET(BINARY 'a'), COLLATION(BINARY 'a');
  2. +---------------------+-----------------------+
  3. | CHARSET(BINARY 'a') | COLLATION(BINARY 'a') |
  4. +---------------------+-----------------------+
  5. | binary              | binary                |
  6. +---------------------+-----------------------+

For operations that combine multiple string inputs and return a single string output, the “aggregation rules” of standard SQL apply for determining the collation of the result:

  • If an explicit COLLATE Y occurs, use Y.

  • If explicit COLLATE Y and COLLATE Z occur, raise an error.

  • Otherwise, if all collations are Y, use Y.

  • Otherwise, the result has no collation.

For example, with CASE ... WHEN a THEN b WHEN b THEN c COLLATE X END, the resulting collation is X. The same applies for UNION, ||, CONCAT(), ELT(), GREATEST(), IF(), and LEAST().

For operations that convert to character data, the character set and collation of the strings that result from the operations are defined by the character_set_connection and collation_connection system variables that determine the default connection character set and collation (see Section 10.4, “Connection Character Sets and Collations”). This applies only to BIN_TO_UUID(), CAST(), CONV(), FORMAT(), HEX(), and SPACE().

An exception to the preceding priniciple occurs for expressions for virtual generated columns. In such expressions, the table character set is used for BIN_TO_UUID(), CONV(), or HEX() results, regardless of connection character set.

If there is any question about the character set or collation of the result returned by a string function, use the CHARSET() or COLLATION() function to find out:

  1. mysql> SELECT USER(), CHARSET(USER()), COLLATION(USER());
  2. +----------------+-----------------+-------------------+
  3. | USER()         | CHARSET(USER()) | COLLATION(USER()) |
  4. +----------------+-----------------+-------------------+
  5. | test@localhost | utf8            | utf8_general_ci   |
  6. +----------------+-----------------+-------------------+
  7. mysql> SELECT CHARSET(COMPRESS('abc')), COLLATION(COMPRESS('abc'));
  8. +--------------------------+----------------------------+
  9. | CHARSET(COMPRESS('abc')) | COLLATION(COMPRESS('abc')) |
  10. +--------------------------+----------------------------+
  11. | binary                   | binary                     |
  12. +--------------------------+----------------------------+

Find a PHP function

Document created the 26/06/2006, last modified the 26/10/2018
Source of the printed document:https://www.gaudry.be/en/mysql-rf-string-functions-charset.html

The infobrol is a personal site whose content is my sole responsibility. The text is available under CreativeCommons license (BY-NC-SA). More info on the terms of use and the author.

References

  1. View the html document Language of the document:en Manuel MySQL : https://dev.mysql.com/

These references and links indicate documents consulted during the writing of this page, or which may provide additional information, but the authors of these sources can not be held responsible for the content of this page.
The author This site is solely responsible for the way in which the various concepts, and the freedoms that are taken with the reference works, are presented here. Remember that you must cross multiple source information to reduce the risk of errors.

Contents Haut