Syntax:
SUBSTRING(str,pos), SUBSTRING(str FROM pos), SUBSTRING(str,pos,len),
SUBSTRING(str FROM pos FOR len)

The forms without a len argument return a substring from string str
starting at position pos. The forms with a len argument return a
substring len characters long from string str, starting at position
pos. The forms that use FROM are standard SQL syntax. Beginning with
MySQL 4.1.0, it is possible to use a negative value for pos. In this
case, the beginning of the substring is pos characters from the end of
the string, rather than the beginning. A negative value may be used for
pos in any of the forms of this function.
