COLLATION 'utf8_general_ci' is not valid for CHARACTER SET 'binary'?

Question

mysql> SELECT LOCATE("n", "München") COLLATE utf8_general_ci;
ERROR 1253 (42000): COLLATION 'utf8_general_ci' is not valid for CHARACTER SET 'binary'

How do I get rid of this error?

What I already tried (copy&paste):

$ mysql -u admin -p $DATABASE
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Welcome to the MySQL monitor.  Commands end with ; or \g.
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mysql> SELECT LOCATE("n", "München") COLLATE utf8_general_ci;
ERROR 1253 (42000): COLLATION 'utf8_general_ci' is not valid for CHARACTER SET 'binary'
mysql> SET NAMES utf8;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT LOCATE("n", "München") COLLATE utf8_general_ci;
ERROR 1253 (42000): COLLATION 'utf8_general_ci' is not valid for CHARACTER SET 'binary'
mysql> SELECT LOCATE(_utf8"n", _utf8"München") COLLATE utf8_general_ci;
ERROR 1253 (42000): COLLATION 'utf8_general_ci' is not valid for CHARACTER SET 'binary'
mysql> SHOW VARIABLES LIKE "character_set_database";
+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| character_set_database | utf8  |
+------------------------+-------+
1 row in set (0.00 sec)

Answer 1

Possibly the server has been compiled with a default character set of binary, so that string literals are being interpreted as such, or the client is set to use a binary mode when communicating with the server. You can change the client and connection character set by calling SET NAMES utf8 (though this is not recommended if your SQL statements are being issued from PHP, for example, as PHP will have its own commands for setting the connection character set). See Connection Character Sets and Collations in the MySQL reference manual.

Alternatively you can use "introducers" to specify explicitly the charset used for the string literals in your LOCATE function, for instance:

LOCATE(_utf8"n", _utf8"München")

See the reference manual page Character String Literal Character Set and Collation for more details.

Answer 2

The COLLATE in my example sets the collation of the return value of LOCATE, the result of which is of type binary.

To set the collation of the arguments:

mysql> SELECT LOCATE(_utf8"n" COLLATE utf8_general_ci,
                     _utf8"München" COLLATE utf8_general_ci) AS locate;
+--------+
| locate |
+--------+
|      3 |
+--------+
1 row in set (0.00 sec)

My motivation actually was finding out whether MySQL takes the collation into account when searching for the substring. Unfortunately it does not. See the result of the second command:

mysql> SELECT LOCATE(_utf8"ü" COLLATE utf8_general_ci,
                     _utf8"München" COLLATE utf8_general_ci) AS locate;
+--------+
| locate |
+--------+
|      2 |
+--------+
1 row in set (0.00 sec)

mysql> SELECT LOCATE(_utf8"u" COLLATE utf8_general_ci,
                     _utf8"München" COLLATE utf8_general_ci) AS locate;
+--------+
| locate |
+--------+
|      0 |
+--------+
1 row in set (0.00 sec)

Test with a temporary table (collation taken into account in WHERE clause, but not in LOCATE):

mysql> CREATE TEMPORARY TABLE test
       (text VARCHAR(100) CHARACTER SET utf8 COLLATE utf8_general_ci);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO test VALUES("München");
Query OK, 1 row affected (0.00 sec)

mysql> SELECT text FROM test WHERE text LIKE "%u%";
+---------+
| text    |
+---------+
| München |
+---------+
1 row in set (0.00 sec)

mysql> SELECT LOCATE("u", text) AS locate FROM test WHERE text LIKE "%u%"; 
+--------+
| locate |
+--------+
|      0 |
+--------+
1 row in set (0.01 sec)