Troubleshooting “Illegal mix of collations” error in mysql

As you can see in the below image this error well known when mix collection used in the database

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This is generally caused by comparing two strings of incompatible collation or by attempting to select data of different collation into a combined column.

There are ways to resolve this things,

once of them to set all data in one collection

change database collation:

ALTER DATABASE database_name CHARACTER SET utf8 COLLATE utf8_unicode_ci;

change table collation:

ALTER TABLE table_name CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci;

change column collation:

ALTER TABLE `table_name` MODIFY column_name VARCHAR(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci;

Another solution is, to use clause COLLATE allows you to specify the collation used in the query.

Your solution is to specify a shared collation for the two columns within the query. Here is an example that uses the COLLATE clause:

SELECT * FROM table ORDER BY key COLLATE latin1_general_ci;

Another option is to use the BINARY operator:
Your solution might look something like this:

SELECT * FROM `table_name` WHERE BINARY a = BINARY b;

or,

SELECT * FROM `table_name` ORDER BY BINARY a;