Syntax:
STR_TO_DATE(str,format)

This is the inverse of the DATE_FORMAT() function. It takes a string
str and a format string format. STR_TO_DATE() returns a DATETIME value
if the format string contains both date and time parts, or a DATE or
TIME value if the string contains only date or time parts.

The date, time, or datetime values contained in str should be given in
the format indicated by format. For the specifiers that can be used in
format, see the DATE_FORMAT() function description. If str contains an
illegal date, time, or datetime value, STR_TO_DATE() returns NULL. An
illegal value also produces a warning.

URL: http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html

