I found my answer in the MySQL documentation :
The LENGTH
function counts bytes :
mysql> SELECT LENGTH('重庆') ;
+------------------+
| LENGTH('重庆') |
+------------------+
| 6 |
+------------------+
1 row in set (0.00 sec)
The CHAR_LENGTH
function counts characters :
mysql> SELECT CHAR_LENGTH('重庆') ;
+-----------------------+
| CHAR_LENGTH('重庆') |
+-----------------------+
| 2 |
+-----------------------+
1 row in set (0.00 sec)