   STR_TO_DATE(str,format)
This is the reverse function 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 table in the DATE_FORMAT() function
description. All other characters are just taken verbatim, thus not being
interpreted.
If str contains an illegal date, time, or datetime value,
STR_TO_DATE() returns NULL.  Starting from MySQL 5.0.3, an
illegal value also produces a warning.
