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Category: Dates and Numbers

Date and Time Aggregation in T-SQL

Daniel Hutmacher shows how you can aggregate date and time data types in SQL Server:

Because the “Duration” column is a “time(3)” datatype, it doesn’t really aggregate into an average that easily.

Msg 8117, Level 16, State 1, Line 20 Operand data type time is invalid for avg operator.

This may seem a little odd, as time is really a continuum just like any integer or floating-point value, right?

Read on to see how to do this and a warning from Daniel about overflowing. This sort of aggregation is a lot easier to do in R, but you can still do it in T-SQL.

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Sorting by Month in Power BI

Cecilia Brusatori shows how you can sort properly by month using Power BI:

Are your visualizations sorting the month field alphabetically? Don’t worry, I’ve been there when I started with Power BI.

If your model has a Dimension Date Table, first congrats, you’re on the right path in what respects to data model, second, you want to use the Month Name from that table, but you noticed that the visualizations don’t sort this column in the way we all wanted, from Jan to Dec.

Click through to see how.

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Floating Point Math and SQL Server

Bert Wagner has more fun with math in SQL Server:

Years ago I was writing a query for a stacked bar chart in SSRS. The chart intended to show the percentage breakdown of distinct values in a table. For example, the chart would show that value A made up 30% of the rows, B made up 3%, C made up 12% and so on. Since every row had a value, I was expecting the stacked bar chart percentages to add up to 100%

However, in many instances the charts would come up short; instead of a full 100%, the percentages would only add up to 98% or 99%. What was going on?

Bert dives deeply into the topic and then gives us some practical suggestions on how to deal with it.

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Custom Power BI Date and Time Formats

Chris Webb continues a series on Power BI custom formats:

In my last post I showed lots of examples of how Power BI’s new custom format string feature can be used to format numbers. This post, looking at dates and times, will be a bit different for two reasons: there are a lot more useful examples of custom date and time formats built into Power BI Desktop, and some of the format placeholders listed in the VBA documentation aren’t supported in Power BI. As a result I’m going to concentrate on some useful formats that aren’t covered well by the examples and highlight a few things that aren’t possible right now.

Read on for a slew of demos.

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PARSE, CAST, and CONVERT

Max Vernon gives us three ways to change data types:

PARSE provides a mechanism to convert a wide variety of character based dates into a datetime data type. From the Docs:

Returns the result of an expression, translated to the requested data type in SQL Server.

Use PARSE only for converting from string to date/time and number types. For general type conversions, continue to use CAST or CONVERT. Keep in mind that there is a certain performance overhead in parsing the string value.

PARSE has its value, but comes at a cost, as Max shows.

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Two Takes on Power BI Time Dimensions

Brett Powell gives us a version of a time dimension in Power Query:

Like most dimensions in a data model, users will generally begin their analysis of time at a high level (e.g. hour) and then filter and drill their way into greater levels of detail, often leveraging hierarchies built into the model and/or drilling functionality in tools like Power BI. To support analysis by multiple grains and hierarchies, the query produces columns which group the seconds into hourly quartiles, hours, and minutes.

Gilbert Quevauvilliers has a script to generate a time dimension as well:

Below is the syntax to create a Time Dimension Table in Power Query

I had a requirement where I needed to create a Time Dimension for a customer. Most of the time I only need the date. Upon searching I could not find a resource where they had created the time dimension only using Power Query.

Gilbert’s gives you a key insight into the value of time dimensions: breaking everything out into periods. 5-minute intervals, 15-minute intervals, or however the company looks at data.

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Building SQL Agent Dates and Times

Kenneth Fisher goes over one of the things in SQL Agent which make me shudder:

Occasionally I’ve seen date and time stored separately as integers. This had some practical applications back before we had date and time data types but there’s still lots of legacy code out there that use them (I’ll give you a really really common example in just a minute).

Unfortunately, you can’t convert datetime to date and time ints directly but it isn’t all that difficult.

Kenneth notes the function you can use as well as a quick query to calculate duration.

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Finding Gaps in Dates

Jason Brimhall shows how you can find gaps in your data:

This method is the much maligned recursive CTE method. In my testing it runs consistently faster with a lower memory grant but does cause a bit more IO to be performed. Some trade-off to be considered there. Both queries are returning the desired data-set which happens to be my missing question days. Only, I have added an extra output in the second query to let me know the day of the week that the missing question occurred on. Maybe I forgot to enter it because it was a weekend day or maybe I opted to not create one at all because the day lands on a Holiday. Let’s take a small peek at the results.

This is a good use for tally tables (or for a calendar table, which is basically a date dimension called something else so you can feel comfortable dropping in a non-warehouse system).

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Getting Prior Year’s Year-To-Date with DAX

Kasper de Jonge takes a look at how to calculate a prior year’s year-to-date over the same period as the current year:

Well maybe.. what happens here is that the DAX engine took the whole date range we have in context and shifts it back 12 months. This means for year 2019 it will use January first to December 31. So we get the entire year, is that what we want? Or do we want to see the sales for the previous year until the day we have data for this year so we can compare? Both need different DAX so let’s take a look.

Read on for a detailed analysis, including where you might go wrong.

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SARGability and Date Functions

Erik Darling shows why you don’t want to use YEAR() or MONTH() in the WHERE clause when querying a large table:

If you’ve been query tuning for a while, you probably know about SARGability, and that wrapping columns in functions is generally a bad idea.

But just like there are slightly different rules for CAST and CONVERT with dates, the repercussions of the function also vary.

The examples I’m going to look at are for YEAR() and MONTH().

Read the whole thing. Maybe “go to brunch” in the middle of it for maximum effect.

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