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Category: Dates and Numbers

Date Calculation (and Calendar Tables) in SQL Server

Published 2022-09-15 by Kevin Feasel

Aaron Bertrand makes the case for calendar tables:

In a previous tip, Simplify Date Period Calculations in SQL Server, I described how to easily calculate certain dates, like the first day of a given month, quarter, or year. Another common scenario is trying to find the last weekday of the month, or the nth Monday or Wednesday. In this tip, I will show ways to simplify these calculations, both with and without a calendar table.

Click through for Aaron’s approach to the problem. I have a blunter approach in creating an expansive calendar table and using it. You do the heavy lifting one time and are good for life on that server.

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Saving DateOnly and TimeOnly to Databases

Published 2022-07-14 by Kevin Feasel

Hasan Savran gives us some mixed news:

DateOnly and TimeOnly are the new members of .NET family, they are introduced in version 6. Developers have been asking to declare only a date or only a time without using the DateTime object for a long time. These two new functions will make the developer’s life easy for sure. You can declare them easily but saving them to a database can be a challenge. Before we go into those details for saving them, Let’s look at how to declare and use them in the code.

But right now, they aren’t so useful for SQL Server. Which is a shame, as there are obvious mapping candidates : DATE and TIME, respectively. My hope is that the Microsoft.Data.SqlClient library gets an update pretty soon to handle them. But read on to learn how Cosmos DB handles these new types.

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Date-Time Binning in Cosmos DB

Published 2022-07-01 by Kevin Feasel

Hasan Savran bins some data:

I wrote about the Date_Bucket() function in SQL Server a couple weeks ago. Azure Cosmos DB team announced the same functionality with a different name DateTimeBin() function. It works exactly the same with the Date_Bucket() function of SQL Server.

     Cosmos DB version of the function has the same number of parameters. The order is different. All the datatime parameters must be in ISO 8601 format (YYYY-MM-DDThh:mm:ss.fffffffZ)

Read on to see how it works.

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Calculating Weekdays with M

Published 2022-06-06 by Kevin Feasel

Kristyna Hughes calculates weekdays on the fly:

Knowing the days between events is a fairly common reporting request because a lot of reporting is created to track SLA’s (service level agreement) and other KPI’s (key performance indicators). While getting the days between two dates is fairly easy to achieve, they tend to follow up and ask how many week days there are between two timed events. For example, one company may have a SLA to ship an order within three week days of the order being placed or else a discount is applied to the order. In this case, I would highly recommend that the company have software that calculates these days in the background and stores the actual week days between order date and ship date in a database. Unfortunately, many companies create policies like this without considering future reporting needs and these values have to be calculated on the backend.

Click through for the code but be sure to read the note that this is all weekdays, including holidays.

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Rounding Differences in Power BI

Published 2022-04-25 by Kevin Feasel

Marco Russo explains the importance of data types for rounding in Power BI:

In one of the last classrooms I delivered, students were wondering why the results of their formulas were close but not identical to the proposed solution. We quickly identified the problem being an issue of data type conversion already covered in Understanding numeric data type conversions in DAX. However, the issue is interesting as a simpler example to show that different DAX calculations can produce different results because of a different way of rounding numbers!

Read on for Marco’s example.

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Time Zones and Extended Events

Published 2022-03-28 by Kevin Feasel

Tomas Zika answers a question:

I’ve helped answer another question that appeared on the SQL Server Slack:

Are timestamps in XE event files you view in SSMS local or server time?

To test this, I need a server in a different timezone than the client (SSMS). I find the quickest and most easy tool for that to be containers – more specifically, Docker.

Click through for the answer, as well as a few Docker-related incidentals.

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SQL Server and Daylight Savings Time

Published 2022-03-25 by Kevin Feasel

Joe Pollock has to turn the clocks forward:

At the start of Daylight Saving Time (DST), which is this weekend in the UK, the clocks will move forward by one hour at 1am. Apart from the fact that we all lose an hour of our night’s sleep, what effect will this have on your scheduled jobs in SQL Server?

As we saw in my last blog post, we know that the SQL Server engine always carries on regardless of when the clocks change, it knows that this has happened, but nothing unusual occurs in the engine itself. However, SQL Server Agent, which runs scheduled tasks, is not the same, as this directly impacts how it works. 

Read on to see what can happen. Also, this can be relevant when you have unique key constraints on datetime values.

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Dealing with Shift Times

Published 2022-03-17 by Kevin Feasel

Kenneth Fisher knows what time it is:

One of the more interesting jobs I’ve had over the years was for a company that created emergency room software. It was pretty cool software and I learned a lot, both about writing queries in SQL Server and about how a software company can be run. One of the more interesting things in the various reports we created was the concept of shift calculations. In other words, what happened during a given shift.

I’ve had to do something similar (though it was for nurse scheduling rather than emergency rooms). Things get really tricky when you start dealing with 12-hour and 16-hour shifts, tracking overtime, and the like.

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Time and Unit Tests

Published 2022-02-08 by Kevin Feasel

Michael J. Swart says, look at the time!:

A flaky test is a unit test that sometimes passes and sometimes fails. The causes of these flaky tests are often elusive because they’re not consistently reproducible.

I’ve found that unit tests that deal with dates and times are notorious for being flaky – especially such tests that talk to SQL Server. I want to explore some of the reasons this can happen.

As a quick note, if you’re using time in database unit tests, don’t use GETUTCDATE() or GETDATE() or any other function like that. It’s a non-deterministic function. Instead, use specific dates and times. That way, you can explicitly test for the types of things Michael points out.

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