Curious about Table.Group aggregatedColumns function

[hansei]

If you group a table in the GUI

1/ the generated code appears as such

Table.Group(#"Kept First Rows", {"Claim ID"}, {{"MinPrice", each List.Min([Contract Price]), type nullable number}})

2/ even though the argument to the 'each' function is a table, you can rewrite the function like the following and it will always work

Table.Group(#"Kept First Rows", {"Claim ID"}, {{"MinPrice", each List.Min(Record.Field(_, "Contract Price")), type nullable number}})

3/ however, rewriting that same code inside a let/in block raises an error

Table.Group(#"Kept First Rows", {"Claim ID"}, {{"MinPrice", each let n = List.Min(Record.Field(_, "Contract Price")) in n, type nullable number}})

Expression.Error: We cannot convert a value of type Table to type Record.

4/ to work inside the let/in block, you need to change from Record.Field to Table.Column like this

Table.Group(#"Kept First Rows", {"Claim ID"}, {{"MinPrice", each let n = List.Min(Table.Column(_, "Contract Price")) in n, type nullable number}})

I get why #3 generates an error. I'm wondering why #2 does not...

[Anonymous · Accepted solution]

Hi @hansei,

I think this should be related to the let-in structure and _ operator.
In the 'group' function, the _ operator after 'each' keyword mapping to the table type, and can be recognized and evaluated after each keyword but these operations seem not processed in the nested let-in structure. (perhaps related to Lazy and eager evaluation)

Evaluation model - PowerQuery M | Microsoft Docs

For your scenario, you can try to use the following formula to fix this issue:

Table.Group(#"Kept First Rows", {"Claim ID"}, {{"MinPrice", each let n = List.Min(_[Contract Price]) in n,  type nullable number}})

Regards,

Xiaoxin Sheng