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create index ind_age on users(age);

mysql> desc testyy;
+-------------+--------------+------+-----+-------------------+----------------------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+--------------+------+-----+-------------------+----------------------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| username | varchar(50) | NO | MUL | NULL | |
| age | int(11) | YES | MUL | 0 |