Occasionally I come across databases written by other developers who have a complete mix of character sets, and where the database character set does not match the character set of fields within a table. This causes all sorts of nasty errors when trying to use CONCAT or CONCAT_WS. Errors like:

Illegal mix of collations for operation 'concat_ws')

The most common cause is that the database type does not match the field or table types.1. Identify the character set that the fields within the table use with the following sql script:

SELECT

COLUMN_NAME,

CHARACTER_SET_NAME,

COLLATION_NAME

FROM

information_schema.`COLUMNS`

WHERE

table_name = "yourtablenamehere";

 2. Identify the character set that the database uses:

SELECT

default_character_set_name

FROM

information_schema.SCHEMATA

WHERE

schema_name = "solution9ssu";

 These should be the same, however a very common difference is that some developers create the database using utf-8 and the fields imported are in latin1_swedish_ci. This causes the collation error.To solve: alter the character encoding of each table. NB. This can cause all manner of issues, so take a full backup first!

ALTER TABLE yourtablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci;