Illegal mix of collations (utf8_unicode_ci,IMPLICIT) and (utf8_turkish_ci,IMPLICIT) for operation '=' and why?
It would be useful if you can provide your tables definition. However, your error suggests that your are trying to check equality between two columns having different collation. 

One solution is to change collation of one of the columns to match the other collation. Check here - Column Level.

Other solution is to use COLLATE clause (I do not know if this works in mariadb, but it should be there).

Personally, I prefer to avoid COLLATE and harmonize string columns as collation mismatch tend to force lots of COLLATEs (maintenance problems).