Understanding MS SQL Server Date Types

Question

Consider the following:

declare @dt datetime, @dt2 datetime2, @d date
set @dt  = '2013-01-01'
set @dt2 = '2013-01-01'
set @d   = '2013-01-01'

select convert(varbinary, @dt) as dt,
       convert(varbinary, @dt2) as dt2,
       convert(varbinary, @d) as d

Output:

dt                    dt2                     d
------------------    --------------------    --------
0x0000A13900000000    0x07000000000094360B    0x94360B

Now, I already understand from the documentation that datetime has a smaller range, and starts from 1753-01-01, while datetime2 and date use 0001-01-01 as their start date.

What I don't understand though, is that datetime appears to be little-endian while datetime2 and date are big-endian. If that's the case, how can they even be properly sortable?

Consider if I want to know how many integer days are represented by a date type. You would think you could do this:

declare @d date
set @d = '0001-01-31'
select cast(convert(varbinary, @d) as int)

But due to the endianness, you get 1966080 days!

To get the correct result of 30 days, you have to reverse it:

select cast(convert(varbinary,reverse(convert(varbinary, @d))) as int)

Or, of course you can do this:

select datediff(d,'0001-01-01', @d)

But that means internally somewhere it is reversing the bytes anyway.

So why did they switch endianness?

我只关心因为我正在 SQLCLR 中处理自定义 UDT，并且字节的二进制顺序在那里似乎很重要，但这些内置类型似乎更加灵活。SQL Server 是否有内部的东西，每种类型都可以提供它自己的排序算法？如果是这样，有没有办法为我的自定义 UDT 使用它？

另请参阅StackOverflow 上的一个相关（但不同）问题。

Answer 1

SQL Server 不依赖其“自己的”数据类型的二进制顺序。对于 CLR 数据类型，您可以使用 iComparable 接口，但正如 @MattJohnson 提到的，SQL Server 会忽略它：

http://connect.microsoft.com/SQLServer/feedback/details/252230/sqlclr-provide-the-ability-to-use-icomparable-or-a-similar-mechanism-for-udts

Microsoft 没有发布有关如何存储和使用不同数据类型的详细信息。然而，联机丛书明确指出，您不能依赖特定数据类型的特定二进制格式，并且它们使用的格式可能随时更改。因此，最好将 INT 存储为 INT 而不是 VARBINARY，因为在下一个 SP 之后您可能无法再读取数据。

至于排序：大部分SQL Server核心都是用C++编写的。我假设内部使用了类似于 iComparable 的方法。但同样，没有关于此的公开文档。即使是这样，您也可能无法利用它，因为 .NET 和 C++ 之间存在固有的差异。