All three statements below return the same result. So I'm struggling to see what the difference is between using dayofyear, day and weekday. Am I missing something or are they all equivalent?
SELECT DATEADD(dayofyear,1,'20111231')
SELECT DATEADD(day,1,'20111231')
SELECT DATEADD(weekday,1,'20111231')
They all return 2012-01-01 00:00:00.000
In that use, no they return the same. Though "day" feels natural for this use.
The difference comes in (e.g.) DATEPART:
SELECT DATEPART(dayofyear,'20111231') -- 365
SELECT DATEPART(day,'20111231') -- 31
SELECT DATEPART(weekday,'20111231') --7
Here's the MSDN ref on it:
dayofyear , day, and weekday return
the same value.
SELECT DATEADD(dayofyear,1,'20111231')
SELECT DATEADD(day,1,'20111231')
SELECT DATEADD(weekday,1,'20111231')
They are all equivalent.
The date parts month, year, day, hour are all units of measure in their own right. However, dayofyear and weekday are not. Yet you can look at it this way -
the datepart "month" changes when the month changes,
the datepart "hour" changes when the hour changes,
the datepart "dayofyear" changes (by 1) when the day changes,
the datepart "weekday" changes (by 1) when the day changes
the datepart "day" changes (by 1) when the day changes
So you have to argue that the "unit of" dayofyear is equal to one day. To DateAdd a "dayofyear" part, it will increase the date by 1 day.
If you are starting with a date variable or date/time column, you can just add 1 to it, e.g.
dateadd(d, 1, #date) == #date + 1
It won't work with date literals
select '2010-12-12' + 1 -- attempts to add the string '2010-12-12' to 1
One would have to cast/convert
select cast('2010-12-12' as datetime) + 1
But by that stage, you might as well use dateadd, which implicitly casts the 3rd argument
select dateadd(d, 1, '2010-12-12') -- 2010-12-13
select dateadd(d, 1, 40522) -- 2010-12-13
Related
I am trying to write a variable using the dateadd and datediff that shows the last two days of previous month. One variable will be the second to last day of the previous month, the one I am having trouble with. The other will be the last day of the previous month, the one I was able to get. I am using SQL Server.
I've tried looking for it on Stack and I have only seen the last day of the previous month given and NOT the second to last day. I tried learning the dateadd and datediff, (which I still want to do).
This is what I tried so far:
Declare #CurrentMonth as date = '3/1/2019'
Declare #SecLastDayPrevMonth as date = DATEADD(MONTH, DATEDIFF(MONTH, 0, #currentmonth), -2)
Declare #LastDayPrevMonth as date = DATEADD(MONTH, DATEDIFF(MONTH, 0, #currentmonth), -1)
The results I am getting for the seclastdayPrevMonth is 2/28/2019. Instead I would want 2/27/2019
I am also getting 2/28/2019 for lastdayprevmonth which is what I want.
I am writing variables because the current month will change every month, and instead of having to update the other days I need within my query, I want to use variables so I am only updating the current month and everything else is flowing through.
And explanation as to why my dateadd/datediff is wrong and an explanation for why the correct dateadd/datediff is the way it is, will be very helpful
Why not refer to the last day when calculating the second last day? Also, your usage of DATEADD is very weird. The syntax is DATEADD(interval, increment, datetime)
Declare #recmonth as date = '3/1/2019'
Declare #LastDayPrevMonth as date = EOMONTH(DATEADD(MONTH, -1, #RecMonth))
Declare #SecLastDayPrevMonth as date = DATEADD(DAY, -1, #LastDayPrevMonth)
SELECT #SecLastDayPrevMonth, #LastDayPrevMonth
So we can calculate the last day of the previous month by subtracting one month from a date and then calling EOMONTH, which returns the last day of a given month. Then the second last day is just subtracting one day from that.
Yields:
SecLastDayPrevMonth LastDayPrevMonth
------------------- ----------------
2019-02-27 2019-02-28
As to "why", DATEDIFF() takes 3 arguments: datepart (string representation of a specific date part), startdate, enddate (both of which must be convertible to a date-ish object).
0 is essentially SQL's epoch, which, in this case is 1/1/1900. So the difference in months between 0 and 3/1/2019 is (119*12)+2 (+2 because we exclude March, since we aren't calculating a full month) = 1430 months difference.
Then, we are trying to add 2 months to our value. DATEADD() takes 3 arguments: datepart, number, date. But, in the example, you are adding 1430 months to whatever date -2 gets converted to (in this case, I believe it would be 12/30/1899, or 2 days before epoch). So, 1430 months after 12/30/1899 would be 2/30/2019, but February only has 28 days in 2019, so it returns 2/28/2019. In a Leap Year, it probably would return 2/29/2019.
To get your #LastDayPrevMonth and #SecLastDayPrevMonth with only DATEDIFF() and DATEADD(), you just need to change your calculations a little.
First thing you want to do is find the First Day of your Given Month. That can be done with DATEADD(month,DATEDIFF(month,0,#CurrentDate),0). We're essentially using the same thing we used above to calculate the number of months since epoch, but then we are adding those months back to epoch.
Now that we know the First Day of the Given Month, all we have to do is subtract days to get a day from the prior month.
So,
DECLARE #CurrentDate date = '2019-03-15' ; -- Changed it to something in the middle of the month.
DECLARE #FirstDayOfGivenMonth = DATEADD(month,DATEDIFF(month,0,#CurrentDate),0) ; -- 3/1/2019
DECLARE #LastDayOfPrevMonth date = DATEADD(day,-1,#FirstDayOfThisMonth) ; -- 2/28/2019
DECLARE #SecLastDayOfPrevMonth date = DATEADD(day, -2, #FirstDayOfThisMonth) ; -- 2/27/2019
SELECT #LastDayOfPrevMonth AS LDPM, #SecLastDayOfPrevMonth AS SLDPM ;
DECLARE #FourDaysLeftInPrevMonth date = DATEADD(day, -4, #FirstDayOfThisMonth) ; -- 2/25/2019
SELECT #FourDaysLeftInPrevMonth AS FourDaysLeftPrev ;
Granted, since SQL 2012, this can all be accomplished much easier with the EOM() function to get to the last day of a month. But if you only could use the two original functions from your original question, this would be one way to get to your needed values.
My company groups all tasks into individual weeks that end on a Thursday. Thus a task due on 3/20/19 would be grouped into the 3/21 week and tasks due on 3/22 group into the 3/28/19 week.
I'm looking to calculate this field (called duedate_Weekdue) based on an input duedate.
The following works but doesn't seem like the simplest way to do this. Anyone have more elegant methods?
Select
getdate() as duedate,
datepart(yy,getdate()) as duedate_yr,
datepart(ww,getdate()) as duedate_ww,
CASE
When datename(dw,Dateadd(day,1,getdate()))='Thursday' Then Dateadd(day,1,getdate())
When datename(dw,Dateadd(day,2,getdate()))='Thursday' Then Dateadd(day,2,getdate())
When datename(dw,Dateadd(day,3,getdate()))='Thursday' Then Dateadd(day,3,getdate())
When datename(dw,Dateadd(day,4,getdate()))='Thursday' Then Dateadd(day,4,getdate())
When datename(dw,Dateadd(day,5,getdate()))='Thursday' Then Dateadd(day,5,getdate())
When datename(dw,Dateadd(day,6,getdate()))='Thursday' Then Dateadd(day,6,getdate())
When datename(dw,Dateadd(day,0,getdate()))='Thursday' Then Dateadd(day,0,getdate())
END as duedate_Weekdue;
You can reduce that to one line of code that uses a little math, and some SQL Engine trivia.
The answers that depend on DATEPART return non-deterministic results, depending on the setting for DATEFIRST, which tells the SQL Engine what day of the week to treat as the first day of the week.
There's a way to do what you want without the risk of getting the wrong result based on a change to the DATEFIRST setting.
Inside SQL Server, day number 0 is January 1, 1900, which happens to have been a Monday. We've all used this little trick to strip the time off of GETDATE() by calculating the number of days since day 0 then adding that number to day 0 to get today's date at midnight:
SELECT DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()),0)
Similarly, day number 3 was January 4, 1900. That's relevant because that day was a Thursday. Applying a little math to the number of days, and relying on the DATEDIFF function to drop fractions, which it does, this calculation will always return the next Thursday for you:
SELECT DATEADD(DAY, (DATEDIFF(DAY, 3, GETDATE())/7)*7 + 7,3);
Credit to this answer for the assist.
So your final query comes down to this:
Select
getdate() as duedate,
datepart(yy,getdate()) as duedate_yr,
datepart(ww,getdate()) as duedate_ww,
DATEADD(DAY, (DATEDIFF(DAY, 3, GETDATE())/7)*7 + 7,3) as duedate_Weekdue;
If the first day of the week is Sunday, by using the modulo operator %:
cast(dateadd(day, (13 - datepart(dw, getdate())) % 7, getdate()) as date) as duedate_Weekdue
I also applied the casting of the result to date.
Try identifying number of day in week with DATEPART and then adding enough days to go to next thursday:
declare #dt date = '2019-03-22'
declare #weekDay int
SELECT #weekDay = DATEPART(dw, #dt)
if #weekDay <= 5
select DATEADD(day, 5 - #weekDay ,#dt)
else
select DATEADD(day, 12 - #weekDay ,#dt)
I have a column with values like '3rd-Wednesday', '2nd-Tuesday', 'Every-Thursday'.
I'd like to create a column that reads those strings, and determines if that date has already come this month, and if it has, then return that date of next month. If it has not passed yet for this month, then it would return the date for this month.
Expected results (on 4/22/16) from the above would be: '05-18-2016', '05-10-2016', '04-28-2016'.
I'd prefer to do it mathematically and avoid creating a calendar table if possible.
Thanks.
Partial answer, which is by no means bug free.
This doesn't cater for 'Every-' entries, but hopefully will give you some inspiration. I'm sure there are plenty of test cases this will fail on, and you might be better off writing a stored proc.
I did try to do this by calculating the day name and day number of the first day of the month, then calculating the next wanted day and applying an offset, but it got messy. I know you said no date table but the CTE simplifies things.
How it works
A CTE creates a calendar for the current month of date and dayname. Some rather suspect parsing code pulls the day name from the test data and joins to the CTE. The where clause filters to dates greater than the Nth occurrence, and the select adds 4 weeks if the date has passed. Or at least that's the theory :)
I'm using DATEFROMPARTS to simplify the code, which is a SQL 2012 function - there are alternatives on SO for 2008.
SELECT * INTO #TEST FROM (VALUES ('3rd-Wednesday'), ('2nd-Tuesday'), ('4th-Monday')) A(Value)
SET DATEFIRST 1
;WITH DAYS AS (
SELECT
CAST(DATEADD(MONTH,DATEDIFF(MONTH,0,GETDATE()),N.Number) AS DATE) Date,
DATENAME(WEEKDAY, DATEADD(MONTH,DATEDIFF(MONTH,0,GETDATE()),N.Number)) DayName
FROM master..spt_values N WHERE N.type = 'P' AND N.number BETWEEN 0 AND 31
)
SELECT
T.Value,
CASE WHEN MIN(D.Date) < GETDATE() THEN DATEADD(WEEK, 4, MIN(D.DATE)) ELSE MIN(D.DATE) END Date
FROM #TEST T
JOIN DAYS D ON REVERSE(SUBSTRING(REVERSE(T.VALUE), 1, CHARINDEX('-', REVERSE(T.VALUE)) -1)) = D.DayName
WHERE D.Date >=
DATEFROMPARTS(
YEAR(GETDATE()),
MONTH(GETDATE()),
1+ 7*(CAST(SUBSTRING(T.Value, 1,1) AS INT) -1)
)
GROUP BY T.Value
Value Date
------------- ----------
2nd-Tuesday 2016-05-10
3rd-Wednesday 2016-05-18
4th-Monday 2016-04-25
I need manipulate with time in one command. I have this date "15.02.2013" and now i need this time change on this "15.01.2012". Date is dynamic stats for me. i need change time with this functions (GATEDATE, YEAR, MONTH) or others.
INPUT:
"15.02.2013"
I need this output:
"15.01.2012"
Can you help me please?
You can do a double DATEADD on the same value.
I.e.
SELECT DATEADD(year, -1, DATEADD(month, -1, #date_from)) FROM Table1
The inner DATEADD is your original, which subtracts the month. This is then wrapped in a second DATEADD which subtracts the year. Alternatively, if it's always 1 year and one month, you could easily subtract 13 months:
SELECT DATEADD(month, -13, #date_from))
You could even choose to have a computed column in your table using the function to calculate the date.
i want to make a select, where the users birthday(date field) is less than 30 days.
what is the best way to to do it? i tried datediff, but i don't know how to put the year aside.
Thanks
You could just use DATEPART function with dayofyear datepart value.
EDIT: honestly, there is a boundary issue in my previous answer (many thanks to Damien): e.g. 2010-12-25 and 2011-01-07 => the difference should be less then 30 days, but DATEPART(dayofyear, #date) - DATEPART(dayofyear, [Birthday]) < 30 condition would skip this record. So I added an additional contition to my answer:
DATEPART(dy, #d) - DATEPART(dy, [Birthday]) < 30 OR
(
DATEPART(mm, #d) = 12 AND
DATEPART(dy, DATEADD(m, 1, #d)) - DATEPART(dy, DATEADD(m, 1, [Birthday])) < 30
)
it adds one month to the each date in the case when the month part of the first date is December and compares the difference.
A common way is to compose a formatted date, as text, and replace the year with the current year; and parse back into a date. Apply datediff on that.
If you find out datediff returns something negative thus the birthday of this year is in the past, add 1 year, and try again. This is for the time period around New Year.
SELECT *
FROM dbo.CheckBirthDay
WHERE (CASE WHEN YEAR(BirthDay) <= YEAR(CURRENT_TIMESTAMP) THEN DATEDIFF(DD,BirthDay,CURRENT_TIMESTAMP) END < 30 )