ASCII(str)
返回字符串str的最左字符的数值。返回0，如果str为空字符串。返回NULL，如果str为NULL。 ASCII()返回数值是从0到255。
SQL> SELECT ASCII('2');
+---------------------------------------------------------+
| ASCII('2') |
+---------------------------------------------------------+
| 50 |
+---------------------------------------------------------+
1 row in set (0.00 sec)
SQL> SELECT ASCII('dx');
+---------------------------------------------------------+
| ASCII('dx') |
+---------------------------------------------------------+
| 100 |
+---------------------------------------------------------+
1 row in set (0.00 sec)
