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B.4.4.1 Case Sensitivity in String Searches

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For nonbinary strings (CHAR, VARCHAR, TEXT), string searches use the collation of the comparison operands. For binary strings (BINARY, VARBINARY, BLOB), comparisons use the numeric values of the bytes in the operands; this means that for alphabetic characters, comparisons will be case-sensitive.

A comparison between a nonbinary string and binary string is treated as a comparison of binary strings.

Simple comparison operations (>=, >, =, <, <=, sorting, and grouping) are based on each character's “sort value.” Characters with the same sort value are treated as the same character. For example, if e and é have the same sort value in a given collation, they compare as equal.

The default character set and collation are utf8mb4 and utf8mb4_0900_ai_ci, so nonbinary string comparisons are case insensitive by default. This means that if you search with col_name LIKE 'a%', you get all column values that start with A or a. To make this search case-sensitive, make sure that one of the operands has a case-sensitive or binary collation. For example, if you are comparing a column and a string that both have the utf8mb4 character set, you can use the COLLATE operator to cause either operand to have the utf8mb4_0900_as_cs or utf8mb4_bin collation:

  1. col_name COLLATE utf8mb4_0900_as_cs LIKE 'a%'
  2. col_name LIKE 'a%' COLLATE utf8mb4_0900_as_cs
  3. col_name COLLATE utf8mb4_bin LIKE 'a%'
  4. col_name LIKE 'a%' COLLATE utf8mb4_bin

If you want a column always to be treated in case-sensitive fashion, declare it with a case-sensitive or binary collation. See Section 13.1.20, “CREATE TABLE Syntax”.

To cause a case-sensitive comparison of nonbinary strings to be case insensitive, use COLLATE to name a case-insensitive collation. The strings in the following example normally are case-sensitive, but COLLATE changes the comparison to be case insensitive:

  1. mysql> SET NAMES 'utf8mb4';
  2. mysql> SET @s1 = 'MySQL' COLLATE utf8mb4_bin,
  3.            @s2 = 'mysql' COLLATE utf8mb4_bin;
  4. mysql> SELECT @s1 = @s2;
  5. +-----------+
  6. | @s1 = @s2 |
  7. +-----------+
  8. |         0 |
  9. +-----------+
  10. mysql> SELECT @s1 COLLATE utf8mb4_0900_ai_ci = @s2;
  11. +--------------------------------------+
  12. | @s1 COLLATE utf8mb4_0900_ai_ci = @s2 |
  13. +--------------------------------------+
  14. |                                    1 |
  15. +--------------------------------------+

A binary string is case-sensitive in comparisons. To compare the string as case insensitive, convert it to a nonbinary string and use COLLATE to name a case-insensitive collation:

  1. mysql> SET @s = BINARY 'MySQL';
  2. mysql> SELECT @s = 'mysql';
  3. +--------------+
  4. | @s = 'mysql' |
  5. +--------------+
  6. |            0 |
  7. +--------------+
  8. mysql> SELECT CONVERT(@s USING utf8mb4) COLLATE utf8mb4_0900_ai_ci = 'mysql';
  9. +----------------------------------------------------------------+
  10. | CONVERT(@s USING utf8mb4) COLLATE utf8mb4_0900_ai_ci = 'mysql' |
  11. +----------------------------------------------------------------+
  12. |                                                              1 |
  13. +----------------------------------------------------------------+

To determine whether a value will compare as a nonbinary or binary string, use the COLLATION() function. This example shows that VERSION() returns a string that has a case-insensitive collation, so comparisons are case insensitive:

  1. mysql> SELECT COLLATION(VERSION());
  2. +----------------------+
  3. | COLLATION(VERSION()) |
  4. +----------------------+
  5. | utf8_general_ci      |
  6. +----------------------+

For binary strings, the collation value is binary, so comparisons will be case sensitive. One context in which you will see binary is for compression functions, which return binary strings as a general rule: string:

  1. mysql> SELECT COLLATION(COMPRESS('x'));
  2. +--------------------------+
  3. | COLLATION(COMPRESS('x')) |
  4. +--------------------------+
  5. | binary                   |
  6. +--------------------------+

To check the sort value of a string, the WEIGHT_STRING() may be helpful. See Section 12.5, “String Functions and Operators”.


Rechercher dans le manuel MySQL

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