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Frequent Visitor

Merge problems - incorrect expand

Hi all, 


I have the following problem. I have 2 tables, which have the following approximate format:


Table 1 (selection of columns)


Email           Transaction ID  Date              Payment processor   Status


john@gmail  0001                01/02/2018   PaymentYellow          Paid

john@gmail  cd_03A            01/03/2018    PaymentBlack           Paid

john@gmail  cd_03B            01/04/2018    PaymentBlack           Paid

john@gmail  cd_03B            01/04/2018    PaymentBlack           Refund


Table 2 (all columns): 


Email           Transaction ID  Date              Payment processor   Transaction number


john@gmail  0001                01/02/2018   PaymentYellow          0

john@gmail  cd_03A            01/03/2018    PaymentBlack           1

john@gmail  cd_03B            01/04/2018    PaymentBlack           2



The transaction number in table 2 is correctly calculated after the following indexing: 


#"Grouped Rows" = Table.Group(#"Removed Duplicates", {"Email"}, {{"Count", each Table.AddIndexColumn(_,"Index2",0,1), type table}}),
#"Expanded Count" = Table.ExpandTableColumn(#"Grouped Rows", "Count", {"PaymentProcessor", "Transaction Id", "Date", "Index2"}, {"PaymentProcessor", "Transaction ID", "Date", "Transaction number"}),


The problem happens after the merge. After merging based on any combination of keys, trimmed, cleaned, transformed all to text, the transaction number, in around 25% of cases is not correctly transfered from Table 2 to Table 1 (left outer merge, with table 1 being on the left - I've tried multiple join options). No duplicate rows, most numbers get transported, but a significant part of them get incorrectly transported, with no obvious (to me) pattern, until now. 


Please help, I've been losing a lot of sleep over this. 


The expected result would be: 


Table 1 (after merge and expand) 


Email           Transaction ID  Date              Payment processor   Status   Transaction number 


john@gmail  0001                01/02/2018   PaymentYellow          Paid           0

john@gmail  cd_03A            01/03/2018    PaymentBlack           Paid           1

john@gmail  cd_03B            01/04/2018    PaymentBlack           Paid           2

john@gmail  cd_03B            01/04/2018    PaymentBlack           Refund       2




Post Patron
Post Patron

You need to add Table.Buffer to the last step in the Query you are going to merge later into another Query (the query where the expend goes wrong). Same as how you need to Table.Buffer if you want to remove duplicates and always keep the 1st item after a sort.

TL;DR - Table.Buffer() fixed issue

Also had this issue.  Completely incorrect join results after merge (Left outer), with seemingly no sense as to how it was processing join and delivering expanded columns (this was joining on multiple columns)

Attempted numerous fixes:  data types, clean & trim, recreating entire structures from scratch - spent 24hrs on this blinkin issue

As above - added Table.Buffer() to the table which formed the right side of the join (this table was gettings its data from a source table which contained a Group By and index).   It worked!   

Thanks all for tip

I have found a solution which is much, much faster than using Table.Buffer. I got the idea from here:


This can be done after joins or inside groupings.

It doesn't have to be a sort, it can also be something else like adding an index column. There are several M table functions that tell the engine to handle a table in a sorted manner. Below examples of what I did. This performs much faster than using Table.Buffer.


Fix data order for a join. Fix inside the NestedJoin function:

Note: you have to experiment which side of the join you have to fix.


= Table.NestedJoin(Table.Sort(Source, {"Index", Order.Ascending}), {"Index"}, Table1, {"Index"}, "Table1Fixed", JoinKind.LeftOuter)

= Table.NestedJoin(Source, {"Index"}, Table.Sort(Table1, {"Index", Order.Ascending}), {"Index"}, "Table1Fixed", JoinKind.LeftOuter)

= Table.NestedJoin(Table.Sort(Source, {"Index", Order.Ascending}), {"Index"}, Table.Sort(Table1, {"Index", Order.Ascending}), {"Index"}, "Table1Fixed", JoinKind.LeftOuter)


Fix data order for a grouping. Fix inside the Group function:

Note: here I have to sort both the table the AddIndexColumn function and the FirstN function.


= Table.Group(Table2, {"w", "x", "y"}, {{"GroupTable", each Table.AddIndexColumn(Table.Sort(_, {"Index", Order.Ascending}), "FirstValueInColumnZ", Table.FirstN(Table.Sort(_, {"Index", Order.Ascending}), 1)[z]{0}, 1), type table}})





Frequent Visitor

This has also been a huge paint point for me.  


I think the Table.Buffer trick has worked but it really slows down performance for me.  


I may end up doing this in an excel and just pasting the results back into Power BI and moving forward from there.  Very disappointing.

Helper II
Helper II

I had a similar issue. What fixed it for me was adding in a dummy "replace values" step before I expanded the merged table out.

Not applicable

I've recently encounterd a similar problem.  Through my own testing, the error (where values are switched) appears to be triggered when a merge is expanded AFTER a grouping event has been conducted in Power Query. 


I went so far as to limit my data fields to just 1 row and found the error still occurring.  While the error is observed in the merge/expand step - the symptom appears to be triggered when merges are placed after a grouping event.  

(The grouping event is not pictured - and additional transformations were conducted between the grouping event and this pictured merge)  The errors impacted a subset of rows (less than 10%).  Below I've singled out one of the changed rows to try and problem solve what might be causing it. 

Merging 1 Row to 1 RowMerging 1 Row to 1 Row

Pay attention to the values in "First Date"Pay attention to the values in "First Date"Pic4.png


I can reproduce this readily and it seems like a pretty big issue.  I've been using Power BI since 2015. 

I have this problem as well using Power BI Desktop May 2019 Report Server. Group By before merge => random expand results for whole number/integer data type.

any solution?

Hi, try adding an Index column after the "Group by". This seems to work for me.

I got a satisfactory result after including a Table.Buffer() before the SORT().

Frequent Visitor

I have the same problem. All data are wrong.

Frequent Visitor

@nazdravanul Did you find a solution to this? I think I am having the same problem? When I click on a table to preview before expanding the data is correct, but it changes after the expand?

Super User III
Super User III



Please share the link from where i can download your PBI file.

Ashish Mathur
Resident Rockstar
Resident Rockstar

Hi @nazdravanul,


Assuming that you have added a index column, then you only need to select Table 1 and click Merge Queries like below.



The detail steps you could have a reference of my attachment.


Best Regards,


Community Support Team _ Cherry Gao
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

Thank you for the reply. I've been using PowerBI for years, I know how to use the UI to do an Expand. That is not my problem. After expand part of the data is incorrectly ported from Table 2 to Table 1 - see details in my post. 

Hi  @nazdravanul,


I'm not very clear abourt your issue based on your description and I cannot reproduce your issue in my side. Could you share a screenshot about your issue?


If it is convenient, could you share a dummy pbix file which can reproduce this issue, so that we can help further investigate on it? You can upload it to OneDrive or Dropbox and post the link here. Do mask sensitive data before uploading.)


Best Regards,


Community Support Team _ Cherry Gao
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

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