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Basic introduction to utf8mb4

Basic Features

utf8mb4 is a character set encoding in MySQL that can store and process Unicode characters.
The Unicode character set contains almost all characters, including characters, symbols, emoticons, etc. of various languages.

The difference with utf8mb3

version

The utf8mb4 character set is supported in MySQL version 5.5.3 and later.
Prior to this version of MySQL, only the utf8 character set was supported, ie utf8mb3.

coding

In MySQL, the utf8 character set actually only supports UTF-8 encoding up to 3 bytes. This means it cannot store and handle some special characters correctly, like some emoji and some auxiliary characters.
In order to solve the limitations of the utf8 character set, MySQL introduced the utf8mb4 character set. The utf8mb4 character set supports up to 4 bytes of UTF-8 encoding, which can represent a wider range of characters, including some special characters and emoji.

utf8mb4 collation

common collation

utf8mb4_general_ci：
- The default collation is case-insensitive and multilingual collations are considered.
- Under this rule, 'a' and 'A' are considered equal.
utf8mb4_unicode_ci：
- Based on the Unicode Collation Algorithm (UCA) default collation, case insensitive .
- Compared with utf8mb4_general_ci, utf8mb4_unicode_ci is more precise and can correctly sort the characters of various languages.
utf8mb4_bin：
- This collation is a binary-based collation, case-sensitive , and sorts according to the binary value of the characters.
- Under this rule, 'A' will come before 'a'.
utf8mb4_0900_ai_ci：
- Introduced in MySQL 8.0.0, a new collation to support the utf8mb4 character set.
- In versions prior to MySQL 8.0.0, the utf8mb4 character set used the utf8mb4_general_ci collation. However, this sorting rule is not accurate enough for some specific character comparisons, which may cause some sorting and comparison results to be unexpected.
- Based on the collation rules of Unicode Collation Algorithm (UCA) 9.0.0, it is case-insensitive and handles the sorting and comparison of various characters more accurately.

In addition to the common collations mentioned above, MySQL also provides some other collations, such as utf8mb4_unicode_520_ci, utf8mb4_unicode_520_bin, etc. These rules can be selected and used according to specific needs.

default collation

When setting the table's default character set to the utf8mb4 character set but not explicitly specifying a collation:

In MySQL 5.7, the default collation is utf8mb4_general_ci.

In MySQL 8.0, the default collation is utf8mb4_0900_ai_ci.

Compatibility issues

Since the utf8mb4_0900_ai_ci collation is a collation introduced by MySQL 8.0, when a table of MySQL 8.0 is imported to MySQL 5.7 or MySQL 5.6, there will be a problem that the character set cannot be recognized.

[Err] 1273 - Unknown collation: 'utf8mb4_0900_ai_ci'

Solution: Modify the collation of the newly created database or manually modify all the collations in the sql file.

Comparison of utf8mb4_unicode_ci and utf8mb4_general_ci

accuracy:
- The utf8mb4_unicode_ci sorting rule is based on standard unicode for sorting and comparison, can handle special characters, and can sort accurately in various voices.
- The utf8mb4_general_ci collation is not based on standard unicode and cannot handle some special characters.
Performance:
- The utf8mb4_general_ci collation is relatively good in sorting performance;
- The utf8mb4_unicode_ci collation implements complex sorting algorithms for special characters, and its performance is slightly worse.
- In most scenarios, there is no significant performance difference between the two

Server level sort parameter control

collation_server

Cited in MySQL 5.6 collation_serveras a system variable, it is used to specify the default character set collation at the server level.
It defines the default character set collation used when creating new tables

View collation_serverthe value of the current MySQL server:

SHOW VARIABLES LIKE 'collation_server';

The command will return a result set containing collation_serverthe variable named and its corresponding value.

Note :

collation_serveris a server-level variable whose value is set when the MySQL server starts.
Usually configured in configuration files (such as my.cnf or my.ini), restart the MySQL server to take effect.

Default parameter rules

If the value of the parameter collation_database is not specified when the service starts, the value of the parameter collation_server will be inherited by default.
If no collation is specified when creating the database, the value of the parameter collation_database is used by default.