Illegal mix of collations MySQL Error

Question

SET collation_connection = 'utf8_general_ci';

then for your databases

ALTER DATABASE your_database_name CHARACTER SET utf8 COLLATE utf8_general_ci;

ALTER TABLE your_table_name CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci;

MySQL sneaks swedish in there sometimes for no sensible reason.

CONVERT(column1 USING utf8)

Solves my problem. Where column1 is the column which gives me this error.

You should set both your table encoding and connection encoding to UTF-8:

ALTER TABLE keywords CHARACTER SET UTF8; -- run once

and

SET NAMES 'UTF8';
SET CHARACTER SET 'UTF8';

Use following statement for error

be careful about your data take backup if data have in table.

 ALTER TABLE your_table_name CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci;

I had my table originally created with CHARSET=latin1. After table conversion to utf8 some columns were not converted, however that was not really obvious. You can try to run SHOW CREATE TABLE my_table; and see which column was not converted or just fix incorrect character set on problematic column with query below (change varchar length and CHARSET and COLLATE according to your needs):

 ALTER TABLE `my_table` CHANGE `my_column` `my_column` VARCHAR(10) CHARSET utf8 
 COLLATE utf8_general_ci NULL;

I found that using cast() was the best solution for me:

cast(Format(amount, "Standard") AS CHAR CHARACTER SET utf8) AS Amount

There is also a convert() function. More details on it here

Another resource here

In general the best way is to Change the table collation. However I have an old application and are not really able to estimate the outcome whether this has side effects. Therefore I tried somehow to convert the string into some other format that solved the collation problem. What I found working is to do the string compare by converting the strings into a hexadecimal representation of it's characters. On the database this is done with HEX(column). For PHP you may use this function:

public static function strToHex($string)
{
    $hex = '';
    for ($i=0; $i<strlen($string); $i++){
        $ord = ord($string[$i]);
        $hexCode = dechex($ord);
        $hex .= substr('0'.$hexCode, -2);
    }
    return strToUpper($hex);
}

When doing the database query, your original UTF8 string must be converted first into an iso string (e.g. using utf8_decode() in PHP) before using it in the DB. Because of the collation type the database cannot have UTF8 characters inside so the comparism should work event though this changes the original string (converting UTF8 characters that are not existend in the ISO charset result in a ? or these are removed entirely). Just make sure that when you write data into the database, that you use the same UTF8 to ISO conversion.