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DATE Columns
The format of a DATE value is 'YYYY-MM-DD'. According to
standard SQL, no other format is allowed. You should use this format in UPDATE
expressions and in the WHERE clause of SELECT statements. For
example:
mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05'; |
As a convenience, MySQL automatically converts a date to a number if
the date is used in a numeric context (and vice versa). It is also smart
enough to allow a "relaxed" string form when updating and in a WHERE
clause that compares a date to a TIMESTAMP, DATE, or a
DATETIME column. (Relaxed form means that any punctuation character
may be used as the separator between parts. For example, '1998-08-15'
and '1998#08#15' are equivalent.) MySQL can also convert a
string containing no separators (such as '19980815'), provided it
makes sense as a date.
The special date '0000-00-00' can be stored and retrieved as
'0000-00-00'. When using a '0000-00-00' date through
MyODBC, it will automatically be converted to NULL in
MyODBC Version 2.50.12 and above, because ODBC can't handle this kind of
date.
Because MySQL performs the conversions described above, the following statements work:
mysql> INSERT INTO tbl_name (idate) VALUES (19970505);
mysql> INSERT INTO tbl_name (idate) VALUES ('19970505');
mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05');
mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00');
mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05';
mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT MOD(idate,100) FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';
|
However, the following will not work:
mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0; |
STRCMP() is a string function, so it converts idate to
a string and performs a string comparison. It does not convert
'19970505' to a date and perform a date comparison.
Note that MySQL does very limited checking whether the date is
correct. If you store an incorrect date, such as '1998-2-31', the
wrong date will be stored.
Because MySQL packs dates for storage, it can't store any given date as it would not fit onto the result buffer. The rules for accepting a date are:
DATE and DATETIME columns.
DATE column and you only know part
of the date.
If the date cannot be converted to any reasonable value, a 0 is
stored in the DATE field, which will be retrieved as
0000-00-00. This is both a speed and convenience issue as we
believe that the database's responsibility is to retrieve the same date
you stored (even if the data was not logically correct in all cases).
We think it is up to the application to check the dates, and not the server.
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