New to SQL Server looking for assistance for a datetime conversion - sql

Question

this is my first submission on Stack Overflow, and I am open to any suggestions on structuring my questions.
I am new to SQL Server, and I have a line of code I that I don't understand.
Can someone please explain this?
declare #prior_year datetime = convert(date, '12/31/' + convert(varchar, datepart(yy, #start_date) - 1))

Answer 1

This is a problematic way to determine the last day of the year before the date(time) stored in some variable.
convert(date, '12/31/' + convert(varchar, datepart(yy, #start_date) - 1))
In English, working outward:
datepart(yy, #start_date)
Tell me the year of the variable #start_date. This is poor form because, if we mean year, we should spell out YEAR (see "Date Parts" in this post). I said this already once today, but this is just laziness, it's like "I'm going to type just enough characters to avoid an unexpected result, but not enough characters to make my intent clear." Worse is YYYY, which I see often - you have a choice between YYYY and YEAR, why not type the one that's actually a word?
datepart(yy, #start_date) - 1
--------------------------- ^^^
Subtract one from that year.I don't have any issues here, but it may be clearer to use an explicit DATEADD() against the variable first, and then extracting the year from that, since things like <some date thing> - 1 can be misread as an attempt to subtract a day (also covered in the shorthand post referenced above).
convert(varchar, datepart(yy, #start_date) - 1))
--^^^^^^^^^^^^^^^^
Convert that explicitly to a string.
Also poor form here because we should always specify the length of variable-width data. In some cases this can lead to unexpected truncation. See this post.
'12/31/' + convert(varchar, datepart(yy, #start_date) - 1))
--^^^^^^^^^^
Prefix that year with 12/31 to produce a mm/dd/yyyy string.
More poor form, because this then assumes the user has US English regional settings, MDY dateformat, etc. If you're going to insist on building a string that represents a date, always use a standard, unambiguous format: YYYYMMDD. (And FWIW YYYY-MM-DD is ambiguous, try it with SET LANGUAGE FRENCH;). See this post.
convert(date, <the rest>)
Converts the whole expression to a date.
A better solution
Ideally we should not be using strings anywhere along the line for any of this. We have built-in functions that provide all kinds of native date handling capabilities without having to worry about languages, regional settings, date format preferences, or string lengths:
SELECT DATEFROMPARTS(YEAR(#start_date) - 1, 12, 31);
-- or, more explicitly:
SELECT DATEFROMPARTS(DATEPART(YEAR, #start_date) - 1, 12, 31);
The other way you can think about this is that the last day of last year is the same day as the day before the first day of this year. Thinking about it in these terms can make it much easier to conceptualize when you are doing things like prior month, where determining the last day of the previous month is more tedious. Or if you are finding endpoints for range queries, because the end of the current reporting period is never as deterministic as the start of the next reporting period. More on that here.
SELECT DATEADD(DAY, -1, DATEFROMPARTS(YEAR(#start_date), 1, 1));
Also of potential use:
Dating Responsibly
Simplify Date Period Calculations in SQL Server
SQL Server DateTime Best Practices
Four short videos in this series

Answer 2

Reading this from the inside out:
We're first finding the year of the date using DATEPART and then subtracting 1 so for a value for today's date: 2020-08-18 we'd be getting an integer value of 19
datepart(yy, '2020-08-18') - 1)
We're then using convert on that value to change it to a varchar:
convert(varchar, 19)
We're then using that new varchar to create a string:
'12/31/' + '19'
Finally we're using convert again to create a date from the string
convert(date, '12/31/19')

Answer 3

Tony,
For completeness please provide the definition and assignment for #start_date. Assuming the statement you provided works, it is probably defined something like
declare #start_date date
set #start_date = '07-01-2020'
Working from the inside out, we can then break the statement down like so ...
This will extract the year value
datepart(yy, #start_date)
This will subtract 1 from the #start_date year value, assuming 2020, this returns 2019
convert(varchar, datepart(yy, #start_date) - 1)
And then this will convert that to the last day of the previous year.
convert(date, '12/31/' + convert(varchar, datepart(yy, #start_date) - 1))
So the statement simply sets the new field to the last day of the prior year.

Answer 4

declare #prior_year datetime = convert(date, '12/31/' + convert(varchar, datepart(yy, #start_date) - 1))
This SQL code declares a #prior_year datetime variable that is hard coded to 12/31/. datepart is used to extract the previous year from another datetime variable #start_date` that is passed in and tacks the returned value onto the end of the '12/31/' hard coded string. So; its really just a formula of 12/31/(#start_date prior year)
So if #start_date is 8/20/2020
You will end up with the output of 12/31/2019
declare #start_date datetime = getdate()
declare #prior_year datetime = convert(date, '12/31/' + convert(varchar, datepart(yy, #start_date) - 1))
select #prior_year
Result: