Looping in Power Query

Question

So, I'm attempting to use Power Query to pull all of the unique Item Types tested the last month, as given below.

let
  Source = Sql.Database("XXX", "YYY"),
  dbo_tblTest = Source{[Schema="dbo",Item="tblTest"]}[Data],
  #"Filtered Rows" = Table.SelectRows(dbo_tblTest, each Date.IsInPreviousNMonths([Test_Stop], 1)),
  #"Added Custom" = Table.AddColumn(#"Filtered Rows", "Custom", each Text.Start([Item],5)),
  #"Removed Duplicates" = Table.Distinct(#"Added Custom", {"Custom"})
in
  #"Removed Duplicates"

I'm getting this.

1. Test_ID --- Item --- Test_Start --- Test_Stop --- Custom
2. 2585048 --- B1846-6-02 --- 1/14/2014 12:46 --- 6/25/2015 14:28 --- B1846
3. 2589879 --- B1843-5-05 --- 12/23/2013 16:46 --- 6/25/2015 14:19 --- B1843
4. 2633483 --- B1907-1-04 --- 8/21/2014 20:47 --- 6/10/2015 6:20 --- B1907
5. 2638786 --- B1361-2-04 --- 6/13/2013 14:21 --- 6/16/2015 14:15 --- B1361
6. 2675663 --- B1345-2-02 --- 5/23/2014 18:39 --- 6/25/2015 21:27 --- B1345

Next, I want to pull the past 10 tests for each of the Item Types listed in Query1, regardless of time period. I figured out how to pull the past 10 tests for the Item Types separately.

let
    Source = Sql.Database("XXX", "YYY"),
    dbo_tblTest = Source{[Schema="dbo",Item="tblTest"]}[Data],
    #"Filtered Rows" = Table.SelectRows(dbo_tblTest, each Text.StartsWith([Item], "B1846")),
    #"Sorted Rows" = Table.Sort(#"Filtered Rows",{{"Test_Stop", Order.Descending}}),
    #"Kept First Rows" = Table.FirstN(#"Sorted Rows",10)
in
    #"Kept First Rows"

I'm getting the following.

1. Test_ID --- Item --- Test_Start --- Test_Stop --- Value
2. 11717643 --- B1846-6-02 --- 7/23/2015 12:48 --- 7/23/2015 12:57 --- 43725341
3. 11716432 --- B1846-1-21 --- 7/23/2015 10:23 --- 7/23/2015 10:29 --- 43724705
4. 11715802 --- B1846-1-21 --- 7/23/2015 9:28 --- 7/23/2015 10:29 --- 43724720
5. 11715505 --- B1846-1-21 --- 7/23/2015 8:59 --- 7/23/2015 9:06 --- 43724675
6. 11715424 --- B1846-1-21 --- 7/23/2015 8:36 --- 7/23/2015 8:59 --- 43724690
7. 11713680 --- B1846-1-55 --- 7/23/2015 5:50 --- 7/23/2015 6:07 --- 43725239
8. 11691169 --- B1846-6-04 --- 7/20/2015 22:47 --- 7/22/2015 20:18 --- 43642835
9. 11690466 --- B1846-6-04 --- 7/20/2015 21:30 --- 7/22/2015 18:41 --- 43642729
10. 11701183 --- B1846-1-140 --- 7/21/2015 21:34 --- 7/21/2015 22:24 --- 43667358
11. 11701184 --- B1846-6-04 --- 7/21/2013 20:35 --- 7/21/2015 20:46 --- 43667359

 But I'm trying to do so in one query. Is it possible?

Answer 1

In Power Query, you often find a higher-order library function that suffices your requirement. If you want to loop- in your case, you'll need "Grouping". It, basically, splits up a table by some key.

You can rewrite your "keep past 10" logic into a function that you apply within each grouped table using Table > TransformColumns > expand the grouped tables back out into one flat table.

Your query should look something like the following:

let
  Source = Sql.Database("XXX", "YYY"),
  dbo_tblTest = Source{[Schema="dbo",Item="tblTest"]}[Data],
  #"Added Custom" = Table.AddColumn(dbo_tblTest, "Custom", each Text.Start([Item],5)),
  #"Grouped Rows" = Table.Group(#"Added Custom", {"Custom"}, {{"Grouped", each _, type table}}),
  Custom2 = Table.TransformColumns(#"Grouped Rows", {{"Grouped", (groupedTable) =>
      let
          #"Sorted Rows" = Table.Sort(groupedTable,{{"Test_Stop", Order.Descending}}),
          #"Kept First Rows" = Table.FirstN(#"Sorted Rows",10)
      in
          #"Kept First Rows"}}),
  #"Removed Other Columns1" = Table.SelectColumns(Custom2,{"Grouped"}),
  #"Expanded Grouped" = Table.ExpandTableColumn(#"Removed Other Columns1", "Grouped", Table.ColumnNames(#"Added Custom"))
in
  #"Expanded Grouped"