T-SQL中的枚举，有可能吗？

All of us know the system function DATEADD function (as an example). this function has 3 parameters which the first one is so strange for me. I looked on the Google so much for finding out the parameter type of the first parameter (which is shown varchar in Object Explorer window), but found nothing!

我们所有人都知道系统函数DATEADD函数（作为例子）。这个函数有3个参数，第一个对我来说很奇怪。我在Google上查找了第一个参数的参数类型（在对象资源管理器窗口中显示为varchar），但没有找到任何内容！

I think it's a kind of Enum used in this function because the usage is very similar to Enum usage:

我认为这是一种在此功能中使用的枚举，因为其用法与枚举用法非常相似：

Print DATEADD(DAY,1,GETDATE()) -- What's that DAY?

My question can be answered if one of this questions be replied:

如果回答其中一个问题，我的问题可以回答：

1. How can I see system functions definition?
2. 如何查看系统功能定义？
3. What's that DAY in above statement and how can I write a function like DATEADD to accept such parameter?
4. 在上面的语句中是什么日子，我如何编写像DATEADD这样的函数来接受这样的参数？
5. If it's an enum, how can I define an enum in TSQL?
6. 如果它是枚举，我如何在TSQL中定义枚举？

1 个解决方案

#1

Day is a type and its values can be:

Day是一种类型，其值可以是：

yy, yyyy: year
qq, q: quarter
mm, m: month
dy, y: day of year
dd, d: day
wk, ww: week
dw, w: weekday
hh: hour
mi, n: minute
ss or s: second
ms: millisecond
mcs: microsecond
ns: nanosecond