User variable name does not support inclusion . and $ characters

[sevenFH]

steps:
dble:

mysql> set @abc=3;
Query OK, 0 rows affected (0.00 sec)

mysql> set @$=5;
ERROR 1064 (HY000): java.sql.SQLSyntaxErrorException: syntax error, expect EQ, actual IDENTIFIER $
mysql> set @.=6;
ERROR 1064 (HY000): java.sql.SQLSyntaxErrorException: error : @
mysql> set @_=7;
Query OK, 0 rows affected (0.00 sec)

mysql> 

mysql:

mysql> set @.=6;
Query OK, 0 rows affected (0.00 sec)

mysql> set @$=5;
Query OK, 0 rows affected (0.00 sec)

mysql>