How does MySQL handle diacritics in character sets and collations?

MySQL Character Set Character Mapping

In MySQL, the default behavior for many Unicode collations, including utf8_general_ci and utf8_unicode_ci, is to map characters with diacritics, such as "åäö," to their base characters without diacritics, such as "aao." This means that queries using diacritic characters may not always produce expected results.

This behavior affects queries in both terminal and PHP contexts. It arises from the specific character encoding and collation rules utilized by MySQL.

Reasons for the Mapping

The mapping of diacritic characters to their base characters is intended to provide a more general and consistent search experience. By treating characters with and without diacritics as equivalents, the database can return results that satisfy a broader range of user queries.

Disabling the Mapping

If you wish to disable this mapping and perform case-sensitive searches while preserving diacritic characters, you can employ the following methods:

• Use a Collation that Preserves Diacritics:
  Switch to a collation that treats characters with and without diacritics differently. An example is utf8_bin, which performs binary comparison of strings.

• Specify Collation for Specific Queries:
  When executing queries, you can specify the collation explicitly using the COLLATE keyword. For instance, you can use the following query to preserve diacritics:

  <code class="sql">select * from topics where name COLLATE utf8_bin = 'Harligt';</code>

Alternatives

If you require case-insensitive searches without the umlaut conversion, you may consider using a full-text index with the ASCII_WS tokenizer. This tokenizer ignores punctuation and diacritics, enabling efficient case-insensitive searches.

Conclusion

MySQL's treatment of characters with diacritics can affect the behavior of search queries. Understanding the default mapping rules and choosing the appropriate collation options is crucial for ensuring that queries accurately reflect the intended search criteria.

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