Illegal mix of collations for operation 'concat'

problem
ERROR 1271 (HY000): Illegal mix of collations for operation 'concat'

as a result of

mysql> show variables like "%character%"; show variables like "%collation%"; +--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
7 rows in set (0.00 sec)

+----------------------+-------------------+
| Variable_name | Value |
+----------------------+-------------------+
| collation_connection | latin1_swedish_ci |
| collation_database | latin1_swedish_ci |
| collation_server | latin1_swedish_ci |
+----------------------+-------------------+
3 rows in set (0.00 sec)

fix it with
set collation_database=utf8_general_ci;
set collation_connection=utf8_general_ci;
set collation_server=utf8_general_ci;