How to Create Custom Collations in MySQL

Creating custom collations in MySQL 8 is a powerful feature that allows database administrators and developers to define how string comparison and sorting are performed. This is particularly useful for languages and character sets where the default collations do not meet specific local or application requirements. This comprehensive guide will walk you through the process of creating and using custom collations in MySQL 8, including several code examples to illustrate the concepts.

Understanding Collations

Before diving into custom collations, it’s essential to understand what a collation is. Collation in MySQL refers to a set of rules that determine how string comparison, sorting, and search are performed. It’s directly linked to the character set, which defines the set of characters recognized by MySQL and their encoding.

Why Custom Collations?

MySQL provides a variety of collations for different character sets, but sometimes the provided collations might not meet specific requirements, such as:

• Sorting rules for local languages.
• Case sensitivity preferences.
• Unique application-specific comparison rules.

Working with Custom Collations in MySQL

Step #1 – Preparation

Before creating a custom collation, ensure your MySQL Server version is 8.0 or above, as custom collations are supported from this version onwards. Also, decide on the character set the collation will apply to, since a collation is always linked to a specific character set.

SHOW CHARACTER SET;

This SQL command lists all character sets available in your MySQL server.

Step #2 – Identify Base Collation

Select a base collation close to your desired rules as a starting point. You will modify this base collation to create your custom one.

SHOW COLLATION WHERE Charset = 'utf8mb4';

Replace ‘utf8mb4’ with the character set you plan to use.

Step #3 – Creating Custom Collation

MySQL does not provide a built-in way to create custom collations directly via SQL queries. Instead, you’ll need to modify the MySQL source code and recompile the server. Here are simplified steps:

1. Download the MySQL Server source code from the official repository.
2. Locate the file defining collations for your chosen character set, usually in the strings/ directory.
3. Copy an existing collation definition and modify it according to your comparison rules. This involves programming in C, focusing on modifying the ctype-charset.c file where charset corresponds to the character set your collation applies to.
4. Recompile MySQL Server with your custom collation included.

This is a labor-intensive and complex process, requiring a good understanding of MySQL’s internal codebase and C programming.

Step #4 – Using Custom Collation

After recompiling MySQL with your custom collation, you can use it when creating tables or querying data:

CREATE TABLE my_table (
  my_column VARCHAR(100) COLLATE my_custom_collation
);

Or, change an existing table’s collation:

ALTER TABLE my_table CONVERT TO CHARACTER SET utf8mb4 COLLATE my_custom_collation;

To compare strings using your custom collation in SQL queries, use:

SELECT 
  * 
FROM 
  my_table 
WHERE 
  my_column COLLATE my_custom_collation = 'some_value';

Considerations and Best Practices

• Backup Your Data: Always backup your data before altering tables’ character sets and collations.
• Testing: Thoroughly test your custom collation in a development environment before deploying it to production. Ensure it accurately compares and sorts data as expected.
• Maintenance: Custom collations mean that you’re diverging from the standard MySQL distribution. This can complicate upgrades, patches, and compatibility with tools and applications.
• Contribution: If your custom collation could be beneficial to others, consider contributing it back to the MySQL community.

Conclusion

Creating custom collations in MySQL 8 is a powerful but complex process, involving modifications to the server’s source code and a deep understanding of how MySQL handles character sets and collations. While it offers the flexibility to suit specific linguistic and application requirements, it comes with challenges such as increased maintenance and testing efforts. However, for those scenarios where default collations do not suffice, custom collations can provide the necessary control over how data is compared and sorted.