DATEDIFF

Randolph West continues a series on covering dates and times, looking at DATEDIFF and DATEDIFF_BIG:

The only functional difference between them is that the DATEDIFF_BIG() returns values as a BIGINT, for results that exceed the boundary of an INT. Keep this in mind when deciding which one to use. For example, the maximum number of seconds an INT can hold is 68 years, while a BIGINT can comfortably store the number of seconds in 10,000 years. This becomes especially important when dealing with microseconds and nanoseconds.

The rest of the post will use DATEDIFF() to refer to both functions.

I think this might be the first time I’d read about DATEDIFF_BIG()and I’m not aware of ever having used it.  But hey, it could make sense if you need to track more than 2 billion microseconds.

Using DATEADD In SQL Server

Randolph West continues a series on date and time types in SQL Server with the DATEADD function:

As with similar functions, DATEADD can do arithmetic on dates as well as times. The syntax is straightforward:

DATEADD (datepart, number, date)

The number portion must be an integer, and it must be within the acceptable range of values for the date part.

Click through for a few examples.

Working With Dates And Times In T-SQL

Tomaz Kastrun walks us through various functions to work with dates and times in T-SQL:

Manipulating date and time in T-SQL is a daily and very common task that every DBA, SQL Developer, BI Developer and data scientist will come across. And over the years, I have accumulated many of the simple date or/and time manipulation combinations of different functions, that it is time, to put them together.

Don’t expect to find here anything you haven’t used or seen – especially, if you are a long time T-SQL developer. The point is to have a post, that will have a lot of examples on date and time manipulation on one place. And by no means, this is not the definite list, but should be quite substantial and the code on Github repository will be update.

The list will be updated on my Github, and therefore this blogpost might not include all. In all of the following examples I will be using function GETDATE() to get the current datetime, unless the examples will have stored dates. Therefore, some of the examples or screen-prints will be different from yours.

This mostly focuses on the DATETIME type rather than DATETIME2 or DATE, but there are a few TIME uses.  Check out Tomaz’s repo for more.

Using AT TIME ZONE In SQL Server

Randolph West looks at the AT TIME ZONE clause when working with a specific time zone in SQL Server:

The time zone name is taken from a list maintained in the following Windows registry hive: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones

Note: For SQL Server on Linux (and Docker containers), a registry shim performs the same function as the Windows registry by intercepting the API calls and returning the expected value(s).

We can also use a Transact-SQL (T-SQL) query against the system view sys.time_zone_info, which uses the information from the registry hive. This is the recommended method if you do not have access to the registry hive.

Click through for a couple of examples.

The Two-Digit Cutoff For Years

Claudio Silva explains why a two-digit year may be interpreted differently in SQL Server versus Excel:

What do you read when you see some date in a format like “01-Jan-00 00:00:00.000”? Keep in mind that I’m talking about the output directly from the table and without any formatting.
1st of January seems to leave no doubt (just because there is no default date format starting with two digits for the year), but…what about the year part ’00’?
It stands for 1900 and the 3rd column is wrong?
Or it stands for 2000 and the DATEPART function is returning the wrong value?

This is why you want to stick with four-digit years.  But if you’re stuck with two-digit years for some reason, Claudio explains how you can get Excel and SQL Server to return the same results.

Creating The Ultimate Calendar Table

Daniel Hutmacher has started to create a comprehensive calendar table:

It’s a collection of inline table value functions that generate different types of calendars, with a number of properties that could be relevant for a calendar dimension. Each function has a unique date column, so you can join the functions you need together in a view or a procedure. The functions are:

  • Dates: a plain gregorian calendar.

  • Fiscal, annual: a gregorian, year-based calendar where you can define the start of a year, like a corporate fiscal calendar.

  • Fiscal, 4-4-5 or 52/53: a week-based calendar where years comprise four quarters of 4+4+5 weeks respectively.

  • Indian national calendar

  • Persian calendar

  • Thai calendar

  • Dates of Catholic and Orthodox easter

  • Lunar cycle

I was going to jokingly be shocked that this list didn’t include the Hebrew or Islamic calendars, but then Daniel had to ruin my fun by explaining why not.  Check it out and when you’re ready to give it a try, head over to his downloads page.

Building A Calendar Table

Louis Davidson has an example of a calendar table in SQL Server:

The solution is part of my calendar/date dimension code, and it is used to do relative positioning over date periods. For example, say you have the need to get data from the 10 days. You can definitely use a simple between to filter the rows, and a bunch of date functions to group by year, month, etc., generally all of the “normal” groupings. But using a calendar table allows you to prebuild a set of date calculations that make the standard values easier to get, and non-standard groupings possible. The technique I will cover makes moving around in the groupings more easily accessible. Like if you want data from the last 3 complete months. The query to do this isn’t rocket science, but it isn’t exactly straightforward either.

For the example, I will use the calendar table that I have on my website here: http://drsql.org/code in the download SimpleDateDimensionCreateAndLoad, and will load it with data up until 2020. Here is that structure:

Read on for examples of usage.  This is an example where thinking relationally differs from thinking procedurally—imagining date ranges as pre-calculated sets isn’t intuitive to procedural developers, but it can give a big performance boost.

DATENAME In SQL Server

Randolph West continues his dates and times series with a new function, DATENAME():

There are many similarities between DATEPART and DATENAME. Where DATEPART returns the date or time part as an integer, DATENAME returns the part as a character string.

This DATENAME function also takes two parameters: the date or time part we want back, and the input date. Just as we saw with DATEPART, the documentation indicates the input date parameter must be an “expression that can resolve to one of the following data types: datesmalldatetimedatetimedatetime2datetimeoffset, or time.”

Similarly, the date and time parts that can be returned look much like those in DATEPART, which gives us another opportunity for the reminder that we should avoid using the available abbreviations in order to help with writing clearly understandable code.

DATENAME is a useful function for displaying parts of dates & times, but Randolph does lay out the caveats.

Using DATEPART In SQL Server

Randolph West shares some thoughts on the DATEPART function:

As we learned some time ago, an INT (integer) in T-SQL uses four bytes, and has a maximum value greater than zero of over 2 billion (there are more than 4 billion values in an integer if we take the negative values into account).

Why then are date and time parts expressed as an INT, which have a lot of overhead for values like 24, 31, and 60?

There are two reasons:

  1. Integers make things simpler. When writing arithmetic in T-SQL and other programming languages, the default data type is usually an integer. Having to memorise which data types are returned from built-in functions becomes unnecessary when we know it will be an INT. Yes, it uses extra memory, but four bytes is a reasonable trade-off against complexity.

  2. One of the return values is nanoseconds. Although DATETIME2(7) only has a granularity down to 100 nanoseconds, DATEPART allows us to return this value, which requires a data type large enough to contain an integer up to 1 billion (nanoseconds can range from 0 to 999,999,900 in increments of 100).

Randolph also explains what happened a few years back to cause iPhone alarms not to fire on January 1st and 2nd.

Removing Time From A DateTime

Wayne Sheffield compares the performance of four methods for removing time from a DateTime data type:

Today, we’ll compare 3 other methods to the DATEADD/DATEDIFF method:

  1. Taking advantage of the fact that a datetime datatype is stored as a float, with the decimal being fractions of a day and the whole numbers being days, we will convert the datetime to float, taking the floor (just the whole numbers), and converting back to datetime.
  2. Using the DATEADD/DATEDIFF routine.
  3. Converting the datetime to DATE and back to datetime.
  4. Converting the datetime to varbinary (which returns just the time), and subtracting that from the datetime value.

While there are other ways of stripping the time (DATETIMEFROMPARTS, string manipulation), those ways are already known as poorly performing. Let’s just concentrate on these four.

Click through for the methods, as well as a performance test to see which is fastest.

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