SqlDateTime.MinValue != DateTime.MinValue，为什么? [英] SqlDateTime.MinValue != DateTime.MinValue, why?
问题描述
我想知道，为什么 SqlDateTime.MinValue 与 DateTime.MinValue 不一样?
I wonder, why SqlDateTime.MinValue is not the same as DateTime.MinValue?
推荐答案
我认为 SQL 和 .NET 的 Date 数据类型之间的区别源于 SQL Server 的 datetime 数据类型，它是最小值和最大值，它的精度比 .NET 的 DateTime 数据类型要古老得多.
I think the difference between SQL's and .NET's Date data types stems from the fact that SQL Server's datetime data type, it's minimum and maximum values, and it's precision are much older than .NET's DateTime datatype.
随着 .NET 的出现，团队决定 Datetime 数据类型应该有一个更自然的最小值，而 01/01/0001 似乎是一个相当合乎逻辑的选择，当然来自 em>编程语言，而不是数据库视角，这个值更自然.
With the advent of .NET, the team decided that the Datetime data type should have a more natural minimum value, and 01/01/0001 seems a fairly logical choice, and certainly from a programming language, rather than database perspective, this value is more natural.
顺便说一句，在 SQL Server 2008 中，有许多新的基于日期的数据类型(日期、时间、DateTime2、DateTimeOffset) 实际上确实提供了更大的范围和精度，并与 .NET 中的 DateTime 数据类型紧密映射.例如，DateTime2 数据类型的日期范围是从 0001-01-01 到 9999-12-31.
Incidentally, with SQL Server 2008, there are a number of new Date-based datatypes (Date, Time, DateTime2, DateTimeOffset) that actually do offer an increased range and precision, and closely map to the DateTime datatype in .NET. For example, the DateTime2 data type has a date range from 0001-01-01 through 9999-12-31.
标准的日期时间"SQL Server 的数据类型的最小值始终为 01/01/1753(而且确实仍然如此！).我必须承认，我也很好奇这个值的意义，所以做了一些挖掘..我发现如下:
The standard "datetime" data type of SQL Server always has had a minimum value of 01/01/1753 (and indeed still does have!). I must admit, I too was curious as to the significance of this value, so did some digging.. What I found was as follows:
从公元 1 年到今天，西方世界实际上使用了两种主要的历法:凯撒大帝的儒略历和教皇格里高利十三世的格里高利历.两种历法的区别仅在于一个规则:决定闰年的规则.在儒略历中，所有能被 4 整除的年份都是闰年.在公历中，所有能被 4 整除的年份都是闰年，但能被 100 整除(但不能被 400 整除)的年份不是闰年.因此，1700、1800 和 1900 年在儒略历中是闰年，但在公历中不是闰年，而 1600 年和 2000 年在两种历法中都是闰年.
During the period between 1 A.D. and today, the Western world has actually used two main calendars: the Julian calendar of Julius Caesar and the Gregorian calendar of Pope Gregory XIII. The two calendars differ with respect to only one rule: the rule for deciding what a leap year is. In the Julian calendar, all years divisible by four are leap years. In the Gregorian calendar, all years divisible by four are leap years, except that years divisible by 100 (but not divisible by 400) are not leap years. Thus, the years 1700, 1800, and 1900 are leap years in the Julian calendar but not in the Gregorian calendar, while the years 1600 and 2000 are leap years in both calendars.
教皇格列高利十三世在 1582 年介绍他的历法时，也指示应跳过 1582 年 10 月 4 日至 1582 年 10 月 15 日之间的日子——也就是说，他说 10 月 4 日之后的第二天应该是 10 月 15 日. 但是，许多国家推迟了转换.英格兰和她的殖民地直到 1752 年才从儒略历转换为公历计算，因此对他们而言，跳过的日期是 1752 年 9 月 4 日至 9 月 14 日.其他国家在其他时间转换，但 1582 年和 1752 年是相关日期我们正在讨论的 DBMS.
When Pope Gregory XIII introduced his calendar in 1582, he also directed that the days between October 4, 1582, and October 15, 1582, should be skipped—that is, he said that the day after October 4 should be October 15. Many countries delayed changing over, though. England and her colonies didn't switch from Julian to Gregorian reckoning until 1752, so for them, the skipped dates were between September 4 and September 14, 1752. Other countries switched at other times, but 1582 and 1752 are the relevant dates for the DBMSs that we're discussing.
因此，当一个人回到很多年前时，日期算术会出现两个问题.第一个是，应该根据儒略规则还是格里高利规则计算转换前的闰年?第二个问题是，什么时候以及如何处理跳过的天数?
Thus, two problems arise with date arithmetic when one goes back many years. The first is, should leap years before the switch be calculated according to the Julian or the Gregorian rules? The second problem is, when and how should the skipped days be handled?
这就是八大 DBMS 处理这些问题的方式:
This is how the Big Eight DBMSs handle these questions:
假装没有开关.这就是 SQL 标准的要求，尽管标准文档不清楚:它只是说日期受到使用公历的日期的自然规则的约束"——无论自然规则"如何.是.这是 DB2 选择的选项.如果假装即使在没有人听说过日历的情况下也始终适用单一日历的规则，则技术术语是预测".日历有效.因此，例如，我们可以说 DB2 遵循一个预兆的公历.
Pretend there was no switch. This is what the SQL Standard seems to require, although the standard document is unclear: It just says that dates are "constrained by the natural rules for dates using the Gregorian calendar"—whatever "natural rules" are. This is the option that DB2 chose. When there is a pretence that a single calendar's rules have always applied even to times when nobody heard of the calendar, the technical term is that a "proleptic" calendar is in force. So, for example, we could say that DB2 follows a proleptic Gregorian calendar.
完全避免这个问题.Microsoft 和 Sybase 将他们的最小日期值设置为 1753 年 1 月 1 日，安全地超过了美国更换日历的时间.这是可以辩护的，但不时会出现抱怨说这两个 DBMS 缺乏其他 DBMS 具有的有用功能，而 SQL 标准需要这些功能.
选择 1582.这就是 Oracle 所做的.Oracle 用户会发现日期算术表达式 1582 年 10 月 15 日减去 1582 年 10 月 4 日产生的值为 1 天(因为 10 月 5-14 日不存在)并且日期 1300 年 2 月 29 日是有效的(因为朱利安闰年年规则适用).当 SQL 标准似乎不需要它时，为什么 Oracle 会遇到额外的麻烦?答案是用户可能需要它.历史学家和天文学家使用这种混合系统而不是预测的公历.(这也是 Sun 在为 Java 实现 GregorianCalendar 类时选择的默认选项——尽管名称如此，但 GregorianCalendar 是一个混合日历.)
Pick 1582. This is what Oracle did. An Oracle user would find that the date-arithmetic expression October 15 1582 minus October 4 1582 yields a value of 1 day (because October 5–14 don't exist) and that the date February 29 1300 is valid (because the Julian leap-year rule applies). Why did Oracle go to extra trouble when the SQL Standard doesn't seem to require it? The answer is that users might require it. Historians and astronomers use this hybrid system instead of a proleptic Gregorian calendar. (This is also the default option that Sun picked when implementing the GregorianCalendar class for Java—despite the name, GregorianCalendar is a hybrid calendar.)
以上引文摘自以下链接:
This above quotation from taken from the following link:
这篇关于SqlDateTime.MinValue != DateTime.MinValue，为什么?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
