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Category: Dates and Numbers

Daylight Savings Time Calculations In Power BI

Published 2019-01-30 by Kevin Feasel

Fred Kaffenberger shows us how to convert UTC to local time zones with daylight savings time:

Quick tip for DST Refresh Date function Power BI Service. I’ll put the code up front, and explain it below. I’ll also say a bit about how to use it at the end. The United States and other places, like Australia, have a pesky thing called Daylight Savings Time. This means that in Central Time US, the offset from Universal Time Coordinated (UTC) is sometimes -6 and other times it’s -5. While Power Query can convert time zones, it doesn’t handle DST. And, my users like to see when the reports were refreshed as a step in evaluating data quality. In 2019, US DST is from March 10 – November 3 (2 AM local time). So, the functions here need to be updated every year.

As promised, here’s the custom function. 

Click through for the custom function and a nice explanation of how it works.

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How Query Store Stores Date Data

Published 2019-01-07 by Kevin Feasel

Erin Stellato shows us a few nuances to the way that Query Store handles dates internally:

Query Store retains query performance data at the plan level.  This data is then broken out into intervals of time, determined by the INTERVAL_LENGTH_MINUTES setting.  The time intervals are found in the sys.query_store_runtime_stats_interval system view, and the start_time and end_time columns are of the DATETIMEOFFSET data type.  This means that the date is time-zone aware, and in Query Store the data is stored as UTC.  Now why does all of this matter?  Because handling dates in Query Store is important if you’re going to query the data directly.

Click through to see a query of what this looks like, as well as a few tips on parsing the data.

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DATEDIFF

Published 2018-11-23 by Kevin Feasel

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.

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Working With Dates And Times In T-SQL

Published 2018-09-07 by Kevin Feasel

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.

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Using AT TIME ZONE In SQL Server

Published 2018-09-06 by Kevin Feasel

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.

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The Two-Digit Cutoff For Years

Published 2018-09-05 by Kevin Feasel

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.

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Creating The Ultimate Calendar Table

Published 2018-07-17 by Kevin Feasel

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.

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Building A Calendar Table

Published 2018-07-13 by Kevin Feasel

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.

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DATENAME In SQL Server

Published 2018-06-21 by Kevin Feasel

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: date, smalldatetime, datetime, datetime2, datetimeoffset, 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.

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