MySQL - How to fix the 'Incorrect string value' error

Posted on Dec 15, 2021

Learn how to fix MySQL 'Incorrect string value' ERROR 1366 when inserting new data

When you try to insert a new record into your MySQL database table, you may encounter an error saying Incorrect string value along with some UTF-8 hex code for the description.

For example, suppose you create a Test table with only one column as follows:

CREATE TABLE `Test` (
  `names` varchar(255)
) 

Next, let’s insert the following Egyptian hieroglyph character into the table:

INSERT INTO Test VALUES('𓀀');

Your MySQL server may respond with the following error:

ERROR 1366 (HY000): 
Incorrect string value: '\xF0\x93\x80\x80' for column 'names' at row 1

The error above is because the character 𓀀 requires 4-bytes to be represented in UTF-8 encoding.

By default, MySQL databases and tables are created using a UTF-8 with 3-bytes encoding. You can see the encoding used for your table by using the SHOW CREATE TABLE statement as follows:

SHOW CREATE TABLE Test \G

Here’s the result from my computer:

*************************** 1. row ***************************
       Table: Test
Create Table: CREATE TABLE `Test` (
  `names` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3

As you can see, the table uses the DEFAULT CHARSET=utf8mb3 and the names column uses CHARACTER SET utf8.

The MySQL utf8 or utf8mb3 can’t store string values that contain a UTF-8 4-bytes character.

To store the values, you need to use the utf8mb4 character set.

Here’s the query to alter your database, table, or column to utf8mb4 character set:

-- Change a database
ALTER DATABASE [database_name] 
  CHARACTER SET = utf8mb4 COLLATE = utf8mb4_unicode_ci; 

-- Change a table
ALTER TABLE [table_name] 
  CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci; 

-- Change a column
ALTER TABLE [table_name] 
  CHANGE [column_name] [column_name] VARCHAR(255) 
  CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

When you change the character set on the database level, then any new table you create for that database in the future will use that character set as the default encoding.

Returning to the Test table, you can alter just the names column to make the INSERT statement works:

ALTER TABLE `Test`
  CHANGE `names` `names` VARCHAR(255) 
  CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

Now you should be able to insert the character 𓁴 into the table:

INSERT INTO Test VALUES('𓁴');
-- Query OK, 1 row affected (0.00 sec)

By default, MySQL version 8 should use utf8mb4 encoding and collation for your databases. If you see utf8 or utf8mb3, then you might be using MySQL version below 8 (MySQL version 5 may default to utf8mb3).

When you encounter this error, pay attention to the characters that you want to insert into the database.

They may look like normal characters, but if you copy and paste them from some source, then they may have a strange encoding attached to them.

For example, the GOTHIC LETTER SAUIL 𐍃 looks like a normal capital S but actually a 4-bytes character:

INSERT INTO Test VALUES('𐍃');

ERROR 1366 (HY000): 
Incorrect string value: '\xF0\x90\x8D\x83' for column 'names' at row 1

Alternatively, you can also pass the hex code (\xF0\x90\x8D\x83 in the example above) into Google to look for the exact character that causes the error.

To conclude, the ERROR 1366: Incorrect string value happens when MySQL can’t insert the value you specified into the table because of incompatible encoding.

You need to modify or remove characters that have 4-bytes UTF-8 encoding, or you can change the encoding and collation used by MySQL.

Note that utf8 in MySQL always refers to utf8mb3.

To use the 4-bytes UTF-8 encoding, it needs to be specified as utf8mb4.

With this information, you should now be able to resolve this error. Feel free to use the provided ALTER statements above if you need it 👍

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