Transform Columns with List.Zip

This post continues series of articles about M Function List.Zip ( first post, second post).

Table.TransformColumns is another function that requires list of pairs if you want to transform several columns in Power Query


When we want to transform all fields we can simply use

Table.TransformColumns(#"Changed Type", {}, Text.Trim) 

// would be great to see same behaviour for Table.TransformColumnTypes

However, when we need to change only part of the table we have to generate list of pairs {column name, function}.

Text.Trim highlighted in last sample is a good example of transformation function.

In general, transformation function can contain any logic and can transform objects of any type.

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How List.Zip helps with Renaming Columns in Power Query

This is my second post about List.Zip. First one was about general usage of List.Zip, where I touched question of transforming column types in Power Query.

Another scenario where List.Zip can be used – renaming columns in Power Query.

When you rename columns manually, auto-generated function looks like

Table.RenameColumns( Source, 
     { {"Column1", "Col 1"}, 
     {"Column2", "Col 2"}, 
     {"Column3", "Col 3"}} )

As well as Table.TransformColumnTypes, it requires list of pairs {“Old Name” , “New Name” } for its second argument.

List.Zip helps to create list of pairs:

Table.RenameColumns( Source, List.Zip( { Table.ColumnNames( Source ), 
    { "Col 1", "Col 2", "Col 3" } } ) )

Result:


Dynamic column names

When we have, so called, “RenamingTable”, which contains two columns, first – with old name, second – with new name, we can use following pattern

Table.RenameColumns( TargetTable, 
    Table.ToColumns( Table.Transpose( RenamingTable ) ), 
    MissingField.Ignore )

You can read more detailed explanation in one of my previous posts.

Using List.Zip, we don’t need RenamingTable, as we can generate new names on the fly by using following pattern

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How to use List.Zip in Power Query

There is one useful Power Query M function – List.Zip, but with poor documentation on MSDN.

I hope, at some point, library of M functions will be available on Github like it is done for VBA. Power Query enthusiasts then would get a chance to contribute. E.g. from MSDN page of Workbook object we can go to Github and make a pull request for changes.

I plan 2-3 posts about application of List.Zip, this is the first one.

How does List.Zip work

Let’s start from “Help” in Power Query editor, it shows simple sample


Note: to get help on function – type name of function in formula bar and press Enter. Pay attention to register, M is case sensitive.

Having short documentation directly in power query editor is great idea! However, it is hard to show all scenarios with function and keep documentation short. In this particular case, it might be not obvious what happens when we have list of lists with more than 2 elements, or lists with different number of elements, or with more than two lists. Continue reading

Power Query Cheat Sheet Update

Just a short post today. I’ve updated my Power Query cheat sheet and created repository on Github so anyone can now contribute.

Good news for Russian-speaking readers. Шпаргалка теперь доступна на русском языке. Ура!

If you want to have this Cheat Sheet in any other language – just translate it and send me a pull request. Too difficult? Then send me an email with translated .docx attached.

Have several ideas for “PQ shortcuts” section, so stay tuned.

Go to Github and get your copy of Power Query cheat sheet.

Power Query cheat sheet repository

How to track refresh time in Power BI Desktop

Usually, I use Power Pivot and VBA in Excel to measure Power Query performance by comparing refresh time.

But, I suppose Power BI Desktop refresh process may be different, therefore would be nice to have something that would allow measure time between start and end of refresh.

Unfortunately, we do not have VBA in Power BI Desktop, nor can trigger and monitor refresh of Power BI Desktop from another application. So, we have only M and queries.

For the experiment I’ve created a new Power BI Desktop file, three queries in it, and put queries in order Start-Delay-End

Start

= DateTime.LocalNow()

Delay

= Function.InvokeAfter(()=> DateTime.LocalNow(), #duration(0,0,0,3))

End

= DateTime.LocalNow()

“Delay” must make a 3-seconds delay. You may read about Function.InvokeAfter in old good post from Chris Webb.

The idea is naive – hope that Power BI Desktop will execute queries in the same order as in the list of queries.

If so – query “Start” will load start time, Delay will make a pause, then “End” will load time of the end of refresh.

However, by default, Power BI Desktop loads tables in parallel, to optimize load time.

This property can be found in Options -> Current File ( Data Load ) -> Enable parallel loading of tables

This really works. I was receiving same time for Start and End tables while this property was enabled.

When I disabled it, I finally got desired difference between Start and End

Then I changed order of queries in the list of queries to check whether it impacts on execution order

Result shows that “Yes – order does matter“:

Of course, Power Query engine must be generating Execution plan when user press Refresh and then follows it. But in simple scenarios, when “Parallel loading of tables” is disabled, seems like Power BI Desktop follows order of queries from Query Editor.

I have no groups of queries, have no references between queries etc. It allowed me to check load time.

What about complex models

I tried to use same technique in more complex file – with groups of queries (but with no references).

I created query “Start” and placed it into the first group.

Similar for “End” query – but in the last group.

Result seems correct

Conclusion

Above is, of course, not a serious solution. Mainly, because you won’t want to disable parallel loading of tables, and won’t rely on order or queries.

Nevertheless, it would be good to have total refresh time directly in the model. It would allow to monitor refresh time of growing datasets.

More sophisticated way of query engine processing analysis is hidden in diagnostics of trace logs. You may read about this in several posts from Chris Webb here, here, and here.

Set of CSVs as a database for Power BI

What if part of your reporting database is a set of CSV files?

Apart from possible problems with different number of columns, data types, delimiters, encoding etc., you have to care about performance.

According to my practice, large number of files kills productivity. It is better to firstly combine CSV / TXT files into one, then use Power Query to load it.

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PowerQuery cheat sheet

Developing queries for Power BI and Power Query I had to look into documentation or check my previous solutions from time to time in order to get answers to same questions again and again. So, I ended up with a creation of cheat sheet for myself. Couldn’t wait more for cheat sheet from Gil Raviv (know, he plans to make it, stay tuned).

My version is not nicely formatted as DAX Reference Card from PowerPivotPro, but still helpful. It helps me with rarely used symbols and data types, and vice versa, frequently used pieces of M code. E.g. Carriage Return symbol in Power Query, get Excel cell value, or work with datetime and duration types etc.

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Shift cells up/down in same column in Power Query

In my previous post I wrote about one interesting technique used by my colleague Zoltán Kaszaki-Krsjak.

Categoty_tmp = Table.AddColumn(Buffer, "Category_tmp", each Buffer[Category]{[ID2]}?),
// It helps to shift values of column [Category] one row upwards.

It is a trick that you most probably will not use in any of your solutions. However, worth to know how it works and how to do this without adding new columns.

“each Buffer[Category]{[ID2]}?” is just a function, so we can combine it with methods described in one of my posts (Transform Column Using Custom Function).

Here is the code.

let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Duplicated Column" = Table.DuplicateColumn(Source, "Category", "Method1"),
#"Duplicated Column1" = Table.DuplicateColumn(#"Duplicated Column", "Category", "Method2"),
#"Duplicated Column2" = Table.DuplicateColumn(#"Duplicated Column1", "Category", "Method3"),
Buffer = Table.Buffer( #"Duplicated Column2" ),

Method1 = Table.FromRecords( Table.TransformRows( Buffer, each [Index = [Index],
    Category = [Category],
    Method1 = Buffer[Method1]{[Index]}?,
    Method2 = [Method2],
    Method3 = [Method3] ] ) ),

Method2 = Table.FromRecords( Table.TransformRows( Method1, (row) =>
    Record.TransformFields( row,
    {"Method2", each Buffer[Method2]{ row[Index] }? } ) ) ),
    // method offered by Miguel Escobar in comment to previous post

Method3 = Table.ReplaceValue(Method2, each [Method3], each Buffer[Method3]{[Index]}?, Replacer.ReplaceValue, {"Method3"})
in
Method3

File with methods is here.

But which method is faster?

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Bar-Mekko chart in Excel with Power Query

Seems, Excel charts is an area that till now wasn’t considered in blogs as a target for Power Query application (Get & Transform in Excel 2016).

Nevertheless, PQ can replace some VBA solutions and make your workbooks macro-free.

In far 2015 my colleague Zoltán Kaszaki-Krsjak shared with me a very good example of how Power Query can help with generation of specific tables for specific charts, which are widely used in our organization.

Idea to write a blog post about this technique became dusty in me OneNote, and probably would wait more if only Jon Peltier hadn’t attracted my attention to this topic again by his recent post.

Sample workbook contains a solution for Bar-Mekko chart (or “variable width column chart”)


Such chart allows to easily see share of categories, growth or absolute value. Can be used to compare market segments or productivity of departments / subsidiaries. Red line in this case shows average growth – another small but important detail.

Interested how to build it?

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Transform Column using custom function in Power Query (part 2)

Year ago I wrote a post Transform table column using own function in Power Query. According to stats, it is quite popular topic, and I’m not surprised.

However, I think that old post is quite complex and too long. Advice should be shorter.

In addition, that post do not cover another very important scenario, when you need to transform column using value from another column.

Consider following case


Task:

  1. We need to Trim column [Name]
  2. We need to divide [Amount] by [Counter] (and do not want to add additional column)

Let’s do this.

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