Syntax:
SUBSTRING_INDEX(str,delim,count)

Returns the substring from string str before count occurrences of the
delimiter delim. If count is positive, everything to the left of the
final delimiter (counting from the left) is returned. If count is
negative, everything to the right of the final delimiter (counting from
the right) is returned. SUBSTRING_INDEX() performs a case-sensitive
match when searching for delim.

URL: http://dev.mysql.com/doc/refman/5.1/en/string-functions.html

