8. The M Language: How to Read a Query

Every Power Query step is a line of M. You can see the whole query in the Advanced Editor (Home › Advanced Editor):

let
    Source = Csv.Document(File.Contents("C:\Data\sales.csv"), [Delimiter=",", Encoding=65001]),
    Headers = Table.PromoteHeaders(Source),
    Typed = Table.TransformColumnTypes(Headers, {{"Amount", type number}}),
    PaidOnly = Table.SelectRows(Typed, each [Status] = "Paid"),
    Totals = Table.Group(PaidOnly, {"City"}, {{"Amount", each List.Sum([Amount]), type number}})
in
    Totals

• let … in: the steps are variables, each one refers to the previous; after in comes the step to return;
• each [Status] = "Paid" — a short function applied to every row, and [Status] is a field of the current row;
• M is case-sensitive: table.selectrows won't work, and "Paid" ≠ "paid";
• parameters (Home › Manage Parameters) replace hard-coded values, such as a folder path or a filter threshold;
• try [Price] * 1 otherwise 0 — catches an error inside an expression.

In the trainer, the formula of each step is shown above the preview; the trainer can't edit M directly.