System functions in SQL Server (1)-date functions

System functions in SQL Server are very useful objects and can help us implement many Algorithm Function. The following is a summary of the built-in functions for system time:
1 . Dateadd:
Usage: dateadd (timeinterval, number, date) adds the number * timeinterval time difference to the date value.
Timeinterval-interval time unit, such as day or DD (day), mm (month), HH (hour), ww (week), YY (year), S (second)
Number ----- number of times of the interval (positive or negative. You can obtain the future time or the past time)
Date -------- reference time
Eg: dateadd (HH, 10, getdate () takes 10 hours after the system time
Dateadd (day,-3, getdate () is the three days before the system time.
2. datediff:
Usage: datediff (timeinterval, date1, date2) calculates the time difference from date1 to date2. The returned value is in the unit of timeinterval. date1 returns a negative value after date2.
Eg: datediff (day, '2014-9-1 ', getdate () returns the number of days from 2009-9-1 to the system time (day)
Datediff (HH, getdate (), '2017-9-1 ') returns the hour from the system time to 2009-9-1 (HH)

3. datename : Return some information in the time and date string (such as year, month ,)
Usage: datename (datepart, date)

Datepart:Yes Date. The following table lists all valid DatepartParameters. The user-defined equivalent variable is invalid.

Datepart	Abbreviations
Year	YY, yyyy
Quarter	QQ, Q
Month	Mm, m
Dayofyear	Dy, y
Day	DD, d
Week	Wk, WW
Weekday	DW
Hour	HH
Minute	Mi, n
Second	SS, S
Millisecond	MS
Microsecond	MCS
Nanosecond	NS
TZOFFSET	TZ

Date; allows expressions, column expressions, User-Defined variables, or string characters. (Original msdn)

example: returns the information about the current system time, such as middle-aged, month, day, dayofyear, and week.
select datename (year, getdate ()
, datename (month, getdate ()
, datename (day, getdate ()
, datename (dayofyear, getdate ()
, datename (weekday, getdate ();
result: 2009 09 2 245 Wednesday

4. datepart
usage: datepart (datepart, date) works similarly to datename.
the difference between the two is shown in the following example: datename returns the string type, datepart returns the int type
select datepart (year, getdate ()
, datepart (month, getdate ()
, datepart (day, getdate ()
, datepart (dayofyear, getdate ()
, datepart (weekday, getdate ();
result: 2009 9 2 245 4