B.4.4.2 Problems Using DATE Columns
The format of a
DATE value is
'. According to standard SQL, no other format is permitted. You should use this format in
UPDATE expressions and in the
WHERE clause of
SELECT statements. For example:
SELECT * FROM t1 WHERE date >= '2003-05-05';
As a convenience, MySQL automatically converts a date to a number if the date is used in numeric context and vice versa. MySQL also permits a “relaxed” string format when updating and in a
WHERE clause that compares a date to a
TIMESTAMP column. “Relaxed” format means that any punctuation character may be used as the separator between parts. For example,
'2004#08#15' are equivalent. MySQL can also convert a string containing no separators (such as
'20040815'), provided it makes sense as a date.
When you compare a
TIMESTAMP to a constant string with the
BETWEEN operators, MySQL normally converts the string to an internal long integer for faster comparison (and also for a bit more “relaxed” string checking). However, this conversion is subject to the following exceptions:
For those exceptions, the comparison is done by converting the objects to strings and performing a string comparison.
To be on the safe side, assume that strings are compared as strings and use the appropriate string functions if you want to compare a temporal value to a string.
The special “zero” date
'0000-00-00' can be stored and retrieved as
'0000-00-00'. When a
'0000-00-00' date is used through Connector/ODBC, it is automatically converted to
NULL because ODBC cannot handle that kind of date.
Because MySQL performs the conversions just described, the following statements work (assume that
idate is a
INSERT INTO t1 (idate) VALUES (19970505); INSERT INTO t1 (idate) VALUES ('19970505'); INSERT INTO t1 (idate) VALUES ('97-05-05'); INSERT INTO t1 (idate) VALUES ('1997.05.05'); INSERT INTO t1 (idate) VALUES ('1997 05 05'); INSERT INTO t1 (idate) VALUES ('0000-00-00'); SELECT idate FROM t1 WHERE idate >= '1997-05-05'; SELECT idate FROM t1 WHERE idate >= 19970505; SELECT MOD(idate,100) FROM t1 WHERE idate >= 19970505; SELECT idate FROM t1 WHERE idate >= '19970505';
However, the following statement does not work:
SELECT idate FROM t1 WHERE STRCMP(idate,'20030505')=0;
STRCMP() is a string function, so it converts
idate to a string in
' format and performs a string comparison. It does not convert
'20030505' to the date
'2003-05-05' and perform a date comparison.
If you enable the
ALLOW_INVALID_DATES SQL mode, MySQL permits you to store dates that are given only limited checking: MySQL requires only that the day is in the range from 1 to 31 and the month is in the range from 1 to 12. This makes MySQL very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation).
MySQL permits you to store dates where the day or month and day are zero. This is convenient if you want to store a birthdate in a
DATE column and you know only part of the date. To disallow zero month or day parts in dates, enable the
MySQL permits you to store a “zero” value of
'0000-00-00' as a “dummy date.” This is in some cases more convenient than using
NULL values. If a date to be stored in a
DATE column cannot be converted to any reasonable value, MySQL stores
'0000-00-00'. To disallow
'0000-00-00', enable the
To have MySQL check all dates and accept only legal dates (unless overridden by
IGNORE), set the
sql_mode system variable to