Forum Discussion
Complex PowerQuery Merge query using substrings
A different way to perform a lookup is to use "Table.SelectRows", although this might be slower - so it is essential that you use the Table.Buffer and for very large table try to partition also. But the basic principle can look as follows:
let Table1 = Table.Buffer(#table({"Column1", "Column2"}, {{1, "4441"}, {2, "4442"}, {3, "4443"}, {4, "443"}})), Table2 = #table({"Column1", "Column2"}, {{1, "4441098"}, {2, "4441097"}, {3, "4441011"}, {4, "4441122"}, {5, "4443111"}, {6, "4443123"}}), Lookup = Table.AddColumn(Table2, "Lookup", (outer) => Table.SelectRows(Table1, each Text.StartsWith(outer[Column2], [Column2]))), #"Expanded Lookup" = Table.ExpandTableColumn(Lookup, "Lookup", {"Column2"}, {"Lookup.Column2"}) in #"Expanded Lookup"Advantage here is that you don't need an exact match, but can use all sorts of conditions, in this case "Text.StartsWith".
This is awesome solution.
I've spend few days trying different ways of achieving same result, but this is by far the best - fastest and cleanest.
I wonder if this could be made into custom function?
Also, is there a solution to speed up subsequent actions after this join? To be honest, performance of PQ after this kind of join falls of the cliff and even simple filter takes ages (and this is for a dimension kind of table, with just about 1000 rows)
The solution below I applied similarly in an Excel / Power Query application I recently developed ( ImkeF knows: the account matching application), which is used in an interactive way, so it had to be fast.
You can add key columns to Table 2, each with the first n (1-7) characters of the value in Column2, acting as key columns. I did so in below query Table2With7Keys.
Next, you can add a column to Table1 with the length of the value in Column2, and group on that length, so you have partitioned tables for each key length.
Now you can merge those nested tables with Table2, using the key column with the corresponding key length.
Then finish up with some expanding and removal.
Query Table2With7Keys:
let
Source = Table2,
#"Added Custom" = Table.AddColumn(Source, "Keys", (x) => Table.FromRows({List.Transform({1..7}, each Text.Middle(x[Column2],0,_))}, List.Transform({1..7}, each "Key"&Text.From(_)))),
#"Expanded Keys" = Table.ExpandTableColumn(#"Added Custom", "Keys", {"Key1", "Key2", "Key3", "Key4", "Key5", "Key6", "Key7"})
in
#"Expanded Keys"
Query MergedTables:
let
Source = Table1,
#"Added Custom" = Table.AddColumn(Source, "Length", each Text.Length([Column2])),
#"Grouped Rows" = Table.Group(#"Added Custom", {"Length"}, {{"AllData", each _, type table}}),
Merged = Table.Buffer(Table.TransformColumns(#"Grouped Rows",{{"AllData", each Table.NestedJoin(_,{"Column2"},Table2With7Keys,{"Key"&Text.From(List.Average(_[Length]))},"Table2Data",JoinKind.LeftOuter), type table}})),
#"Expanded AllData" = Table.ExpandTableColumn(Merged, "AllData", {"Column1", "Column2", "Table2Data"}, {"Column1", "Column2", "Table2Data"}),
#"Expanded Table2Data" = Table.ExpandTableColumn(#"Expanded AllData", "Table2Data", {"Column1", "Column2"}, {"Table2.Column1", "Table2.Column2"}),
#"Removed Columns" = Table.RemoveColumns(#"Expanded Table2Data",{"Length"})
in
#"Removed Columns"