Comparison of time in SQL Server Plus and minus time

Recently in the writing of a project, using the time of comparison and time of the addition and subtraction, here briefly, I was just contact with the database soon, if there are said improper place to hope that you forgive, correct me

A comparison of time

In the beginning, I spent more time with time>getdate () simple rough

In fact, SQL Server has its own time comparison function, so it can be brought with it, but also safe and convenient

DATEDIFF ();

The parsing of the DateDiff function in the W3school I refer to IS as follows:

Grammar

   DATEDIFF (datepart,startdate,enddate)
   Which datepart is only you want to compare this two-time year? Or a month? Or is it a day? What's the difference between hours?
   The specific wording of the reference:

DatePart	Abbreviations
Years	YY, yyyy
Quarter	QQ, Q
Month	MM, M
The day of the year	Dy, y
Day	DD, D
Week	WK, WW
Week	DW, W
Hours	hh
Minutes	MI, n
Seconds	SS, S
Milliseconds	Ms
Subtle	MCs
Na-Sec	Ns

Simply write an example to illustrate the following:

DATEDIFF (HH, ' 2014-06-25 8:00:00 ', ' 2014-06-25 12:58:00 ') gets a value of 4, which is the time difference between two hours compared to 4 hours

If you want to compare the number of days, change the HH to D, other comparisons can be seen

Second, the time of the addition and subtraction

I started with the time+1 is in time to add a day, minus one day with Time-1, the same simple rough

Let's talk about the functions of time-plus-minus in SQL Server

The syntax is as follows:

     DATEADD (datepart,number,date)
     Which datepart is the form of time you want to add and subtract, plus year? Or a month? Or the day, etc.
     The specific wording is as follows:

DatePart	Abbreviations
Years	YY, yyyy
Quarter	QQ, Q
Month	MM, M
The day of the year	Dy, y
Day	DD, D
Week	WK, WW
Week	DW, W
Hours	hh
Minutes	MI, n
Seconds	SS, S
Milliseconds	Ms
Subtle	MCs
Na-Sec	Ns

The feeling is similar to the above DateDiff, see the example directly:

DATEADD (D,1,getdate ()) is the current time plus day, minus one day with DATEADD (D,-1,getdate ())

If it is for the year, month, hour and so on plus minus reference, modify datepart can, convenient and quick

Ps:w3school is still a good http://www.w3school.com.cn/sql/.