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B.4.4.2 Problems Using DATE Columns
The format of a DATE value is
'YYYY-MM-DD'. 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:
As a convenience, MySQL automatically converts a date to a
number if the date is used in a 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 DATE,
DATETIME, or
TIMESTAMP column.
“Relaxed” format means that any punctuation
character may be used as the separator between parts. For
example, '2004-08-15' and
'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 DATE,
TIME,
DATETIME, or
TIMESTAMP to a constant string
with the <, <=,
=, >=,
>, or 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
DATE column):
However, the following statement does not work:
STRCMP() is a string function,
so it converts idate to a string in
'YYYY-MM-DD' 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
NO_ZERO_IN_DATE mode.
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
NO_ZERO_DATE mode.
To have MySQL check all dates and accept only legal dates
(unless overridden by IGNORE), set the
sql_mode system variable to
"NO_ZERO_IN_DATE,NO_ZERO_DATE".
Document created the 26/06/2006, last modified the 26/10/2018
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