Illegal mix of collations for operation ' IN '

[Seo-luntan]

"Illegal mix of collations for operation ' IN '"

What is it and how can we deal with it? I got it from our forum's error logs.
Thanks! 

[MrPhil]

Without more information, I can't tell you for sure, but it sounds like a MySQL error message complaining that your database (or at least, a specific table within it), is using a different encoding and/or collation sequence than some other piece of data that it's being compared against in an "IN" clause. Check all your database tables and their individual fields to see if they have the same "collation" setting, and see if your pages are displaying in the same encoding. The page will be something like "UTF-8", while the tables and fields within tables should be UTF-8 plus some language-specific collation (e.g., utf-8_general_ci). Most likely one table (or field(s) within it) somehow ended up with a different collation than another.