How to change the CHARACTER SET (and COLLATION) throughout a database?

change database collation:

ALTER DATABASE <database_name> CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;

change table collation:

ALTER TABLE <table_name> CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;

change column collation:

ALTER TABLE <table_name> MODIFY <column_name> VARCHAR(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;

What do the parts of utf8mb4_0900_ai_ci mean?

3 bytes -- utf8
4 bytes -- utf8mb4 (new)

v4.0 --   _unicode_
v5.20 --  _unicode_520_
v9.0 --   _0900_ (new)

_bin      -- just compare the bits; don't consider case folding, accents, etc
_ci       -- explicitly case insensitive (A=a) and implicitly accent insensitive (a=á)
_ai_ci    -- explicitly case insensitive and accent insensitive
_as (etc) -- accent-sensitive (etc)

_bin         -- simple, fast
_general_ci  -- fails to compare multiletters; eg ss=ß, somewhat fast
...          -- slower
_0900_       -- (8.0) much faster because of a rewrite

More info:

  • What are the differences between utf8_general_ci and utf8_unicode_ci?
  • What's the difference between utf8_general_ci and utf8_unicode_ci?
  • How to change collation of database, table, column?
  • What's the difference between utf8_general_ci and utf8_unicode_ci?

Adding to what David Whittaker posted, I have created a query that generates the complete table and columns alter statement that will convert each table. It may be a good idea to run

SET SESSION group_concat_max_len = 100000;

first to make sure your group concat doesn't go over the very small limit as seen here.

     SELECT a.table_name, concat('ALTER TABLE ', a.table_schema, '.', a.table_name, ' DEFAULT CHARACTER SET utf8mb4 DEFAULT COLLATE utf8mb4_unicode_ci, ',
        group_concat(distinct(concat(' MODIFY ',  column_name, ' ', column_type, ' CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci ', if (is_nullable = 'NO', ' NOT', ''), ' NULL ',
        if (COLUMN_DEFAULT is not null, CONCAT(' DEFAULT \'', COLUMN_DEFAULT, '\''), ''), if (EXTRA != '', CONCAT(' ', EXTRA), '')))), ';') as alter_statement
    FROM information_schema.columns a
    INNER JOIN INFORMATION_SCHEMA.TABLES b ON a.TABLE_CATALOG = b.TABLE_CATALOG
        AND a.TABLE_SCHEMA = b.TABLE_SCHEMA
        AND a.TABLE_NAME = b.TABLE_NAME
        AND b.table_type != 'view'
    WHERE a.table_schema = ? and (collation_name = 'latin1_swedish_ci' or collation_name = 'utf8mb4_general_ci')
    GROUP BY table_name;

A difference here between the previous answer is it was using utf8 instead of ut8mb4 and using t1.data_type with t1.CHARACTER_MAXIMUM_LENGTH didn't work for enums. Also, my query excludes views since those will have to altered separately.

I simply used a Perl script to return all these alters as an array and iterated over them, fixed the columns that were too long (generally they were varchar(256) when the data generally only had 20 characters in them so that was an easy fix).

I found some data was corrupted when altering from latin1 -> utf8mb4. It appeared to be utf8 encoded latin1 characters in columns would get goofed in the conversion. I simply held data from the columns I knew was going to be an issue in memory from before and after the alter and compared them and generated update statements to fix the data.


Here's how to change all databases/tables/columns. Run these queries and they will output all of the subsequent queries necessary to convert your entire database to character encoding utf8mb4 and collations to the MySQL 8 default of utf8mb4_0900_ai_ci. Hope this helps!

-- Change DATABASE Default Collation

SELECT
CONCAT('ALTER DATABASE `', SCHEMA_NAME,'` CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;')
FROM information_schema.SCHEMATA
WHERE SCHEMA_NAME
NOT IN ('sys','mysql','information_schema','performance_schema','innodb')
AND SCHEMA_NAME LIKE 'database_name';

Note that changing the schema default changes the default for new tables (and their columns). It does not modify existing columns of existing tables.

-- Change TABLE Collation / Char Set

SELECT
CONCAT('ALTER TABLE `', TABLE_SCHEMA, '`.`', TABLE_NAME, '` CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;')
FROM information_schema.TABLES
WHERE TABLE_SCHEMA
NOT IN ('sys','mysql','information_schema','performance_schema','innodb')
AND TABLE_TYPE = 'BASE TABLE'
AND TABLE_SCHEMA LIKE 'database_name';

Note that changing the table default changes the default for new columns. It does not modify existing columns on existing tables.

-- Change COLUMN Collation / Char Set

SELECT
CONCAT('ALTER TABLE `', TABLE_SCHEMA, '`.`', TABLE_NAME, '` MODIFY COLUMN `', COLUMN_NAME, '` ', COLUMN_TYPE,
              IF(COLUMN_DEFAULT IS NOT NULL, CONCAT(' DEFAULT \'', COLUMN_DEFAULT, '\''), ''),
              IF(IS_NULLABLE = 'YES', ' NULL ', ' NOT NULL '),
              ' COLLATE utf8mb4_0900_ai_ci;')
FROM information_schema.COLUMNS
WHERE TABLE_SCHEMA
NOT IN ('sys','mysql','information_schema','performance_schema','innodb')
AND COLLATION_NAME IS NOT NULL
AND TABLE_SCHEMA LIKE 'database_name'
AND COLLATION_NAME = 'old_collation_name';

This changes the actual columns and the database behaviour on queries. It does not, however, convert the data if the data is not in a compatible collation/character set. See https://dev.mysql.com/blog-archive/mysql-8-0-collations-migrating-from-older-collations/ for details on migrating from older collations. We also assume here that your default values do not include a single quote - it would need to be escaped - and we ensure that COLLATION_NAME is not NULL to exclude columns with integers, timestamps etc.

We filter out the built-in system schemas such as sys and mysql in all three cases, as these should likely not be modified unless you have an explicit reason to do so.


Beware that in Mysql, the utf8 character set is only a subset of the real UTF8 character set. In order to save one byte of storage, the Mysql team decided to store only three bytes of a UTF8 characters instead of the full four-bytes. That means that some east asian language and emoji aren't fully supported. To make sure you can store all UTF8 characters, use the utf8mb4 data type, and utf8mb4_bin or utf8mb4_general_ci in Mysql.

Tags:

Mysql

Sql

Collation

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