sql - Datepart 和 @@datefirst 的区别

Question

据我了解，DateFirst 是一周的第一天，或者一周开始的那一天。但是，当我运行这些查询时，我的结果与我认为的理解不符

select @@datefirst,DATENAME(WEEKDAY,@@datefirst)
SELECT DATEPART(WEEKDAY,'20190113'),DATENAME(WEEKDAY,'20190113')
SELECT DATEPART(WEEKDAY,'20190114'),DATENAME(WEEKDAY,'20190114')
SELECT DATEPART(WEEKDAY,'20190115'),DATENAME(WEEKDAY,'20190115')
SELECT DATEPART(WEEKDAY,'20190116'),DATENAME(WEEKDAY,'20190116')
SELECT DATEPART(WEEKDAY,'20190117'),DATENAME(WEEKDAY,'20190117')
SELECT DATEPART(WEEKDAY,'20190118'),DATENAME(WEEKDAY,'20190118')
SELECT DATEPART(WEEKDAY,'20190119'),DATENAME(WEEKDAY,'20190119');

我明白了

7 Monday (this is the @@datefirst line)
1 Sunday
2 Monday
3 Tuesday
4 Wednesday
5 Thursday
6 Friday
7 Saturday

@@datefirst 如何给我 7，而这一天给我星期一，而 7 是星期六？这是配置问题还是我的误解？

Answer 1

基于以下Microsoft Documentation :

This function returns a character string representing the specified datepart of the specified date.

Syntax: DATENAME ( datepart , date )

<小时/>

更新 1

在同一篇文章的以下部分中:为不在日期参数中的日期部分返回默认值

If the data type of the date argument does not have the specified datepart, DATENAME will return the default for that datepart only if the date argument has a literal .

For example, the default year-month-day for any date data type is 1900-01-01. This statement has date part arguments for datepart, a time argument for date, and DATENAME returns 1900, January, 1, 1, Monday

因此，当您输入数字7时，输入将被视为1900-01-01 07:00:00，返回星期一

如果您转到 SQL Server 并运行以下命令

select DATENAME(WEEKDAY,'1900-01-01 07:00:00')

返回

Monday