Solving MySQL Error 1271: A Guide to Fixing Illegal Mix of Collations

MySQL Error 1271 – SQLSTATE: HY000 (ER_CANT_AGGREGATE_NCOLLATIONS) is an error that arises when an operation involves a combination of different collations that are not compatible. The error message “Illegal mix of collations for operation ‘%s'” suggests that the collations of the operands do not match and cannot be reconciled for the given operation, which could be a string comparison, concatenation, or a function like GROUP BY.

Understanding the Error

Collations in MySQL dictate how string comparison is performed and are closely tied to character sets, where each character set has one or more collations. Error 1271 occurs when you try to perform an operation on two or more strings that have different collations, and MySQL cannot determine which collation to use for the operation.

Diagnosing the Problem

To fix this error, you need to identify where the collation mismatch is occurring:

  1. Check Collations of Individual Columns: Use SHOW FULL COLUMNS FROM table_name; to see the collation of each column.
  2. Determine Collation of Expressions: If you’re dealing with expressions or literals, you might need to use the COLLATION() function to find out their collation.
  3. Examine Collation of the Database and Tables: Use SHOW CREATE DATABASE db_name; and SHOW CREATE TABLE table_name; to see the default collations.

Fixing the Error

Once you have identified where the mismatch is, you can take several steps to correct it:

  1. Converting Collations in a Query: You can convert collations on-the-fly in a query using the COLLATE clause. For example, if you’re comparing two columns with different collations: SELECT * FROM table1 a JOIN table2 b ON a.string_column COLLATE utf8_general_ci = b.string_column COLLATE utf8_general_ci; This sets both columns to use the utf8_general_ci collation for the operation.
  2. Changing Column Collation: To permanently change the collation of a column, you can use ALTER TABLE: ALTER TABLE table_name MODIFY column_name VARCHAR(255) COLLATE utf8_general_ci; Adjust the column type and size as needed.
  3. Setting the Default Collation of Database and Tables: To avoid future issues, you can set the default collation for your database and tables: ALTER DATABASE db_name COLLATE utf8_general_ci; ALTER TABLE table_name COLLATE utf8_general_ci;
  4. Using Binary Collation as a Last Resort: If you want to perform operations without considering collation, you can use binary collation:
    sql SELECT * FROM table1 a JOIN table2 b ON a.string_column COLLATE utf8_bin = b.string_column COLLATE utf8_bin;
    This compares strings byte by byte.


  • Ensure that changing collations does not lead to data loss, especially when modifying columns with existing data.
  • Be consistent with the use of collations across your database to avoid similar issues.
  • When designing your database, choose a collation that supports all the languages you need to store to minimize the need for changes later on.

By understanding how collations work in MySQL and taking appropriate measures to reconcile them, you can effectively resolve Error 1271 and ensure smooth operation of your database. Always remember to backup your database before making structural changes.

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