Analysis of error reports caused by different MySQL table sorting rules

The error when joining MySQL multiple tables is as follows: [Err]1267 – Illegal mix of collations(utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation '=

means two tables The collation (COLLATION) is different and the comparison cannot be completed. COLLATION is used for sorting and size comparison. A character set has one or more COLLATIONs and ends with _ci (case insensitive), _cs (case sensitive) or _bin (binary). When doing comparisons, you should ensure that the character ordering of both tables is the same. Generally, you don't specify when creating a table. You can use the default. If it is all the default, there will be no problem. This article mainly introduces the analysis of different errors in MySQL table sorting rules. I hope it can help everyone.

Let’s simulate various scenarios. The table structure is as follows (the default sorting rule of utf8 is utf8_general_ci):

mysql> show create table test.cs\G
*************************** 1. row ***************************
    Table: cs
Create Table: CREATE TABLE `cs` (
 `id` int(11) DEFAULT NULL,
 `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.01 sec)

View the table default sorting rule Set

mysql> select TABLE_SCHEMA,TABLE_NAME,TABLE_COLLATION from information_schema.tables where table_name='cs';
+--------------+------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | TABLE_COLLATION |
+--------------+------------+-----------------+
| test     | cs     | utf8_general_ci |
+--------------+------------+-----------------+
1 row in set (0.00 sec)

View column sorting rule set

mysql> select TABLE_SCHEMA,TABLE_NAME,COLUMN_NAME,COLLATION_NAME from information_schema.COLUMNS where TABLE_NAME='cs';
+--------------+------------+-------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | COLUMN_NAME | COLLATION_NAME |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | NULL      |
| test     | cs     | name    | utf8_general_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

Upgrading from utf8 to utf8mb4 does not support online ddl. As follows:

##

mysql> ALTER TABLE cs CONVERT TO CHARACTER SET utf8mb4,ALGORITHM=INPLACE,LOCK=NONE;
ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

Changing from utf8.utf8_general_ci to utf8.utf8_unicode_ci does not support online ddl, as follows:

mysql> ALTER TABLE cs CONVERT TO CHARACTER SET utf8 collate utf8_unicode_ci,ALGORITHM=INPLACE,LOCK=NONE;
ERROR 1846 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY.

If you use the following method to modify the character set, you will find that only the table level is changed, not the column level.

mysql> ALTER TABLE cs CHARACTER SET utf8 collate utf8_unicode_ci;      
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
 
mysql> select TABLE_SCHEMA,TABLE_NAME,TABLE_COLLATION from information_schema.tables where table_name='cs';      
+--------------+------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | TABLE_COLLATION |
+--------------+------------+-----------------+
| test     | cs     | utf8_unicode_ci |
+--------------+------------+-----------------+
1 row in set (0.00 sec)
 
mysql> select TABLE_SCHEMA,TABLE_NAME,COLUMN_NAME,COLLATION_NAME from information_schema.COLUMNS where TABLE_NAME='cs';
+--------------+------------+-------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | COLUMN_NAME | COLLATION_NAME |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | NULL      |
| test     | cs     | name    | utf8_general_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

So don’t forget to add CONVERT TO when you actually change the character set, as follows:

mysql> ALTER TABLE cs CONVERT TO CHARACTER SET utf8 collate utf8_unicode_ci;
Query OK, 5 rows affected (0.06 sec)
Records: 5 Duplicates: 0 Warnings: 0
 
mysql> select TABLE_SCHEMA,TABLE_NAME,COLUMN_NAME,COLLATION_NAME from information_schema.COLUMNS where TABLE_NAME='cs';
+--------------+------------+-------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | COLUMN_NAME | COLLATION_NAME |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | NULL      |
| test     | cs     | name    | utf8_unicode_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

To To only change the default character set of a table, you should use this statement:

mysql> ALTER TABLE cs default CHARACTER SET utf8 collate utf8_general_ci,ALGORITHM=INPLACE,LOCK=NONE;         
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
 
mysql> select TABLE_SCHEMA,TABLE_NAME,TABLE_COLLATION from information_schema.tables where table_name='cs';      
+--------------+------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | TABLE_COLLATION |
+--------------+------------+-----------------+
| test     | cs     | utf8_general_ci |
+--------------+------------+-----------------+
1 row in set (0.00 sec)
 
mysql> select TABLE_SCHEMA,TABLE_NAME,COLUMN_NAME,COLLATION_NAME from information_schema.COLUMNS where TABLE_NAME='cs';  
+--------------+------------+-------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | COLUMN_NAME | COLLATION_NAME |
+--------------+------------+-------------+-----------------+
| test     | cs     | id     | NULL      |
| test     | cs     | name    | utf8_unicode_ci |
+--------------+------------+-------------+-----------------+
2 rows in set (0.00 sec)

You can find that the column character set has not changed, and only new columns will inherit the characters of the table by default. set(utf8.utf8_general_ci).

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