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Dicken's avatar
Dicken
Icon for Post Prodigy rankPost Prodigy
4 months ago
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Transform columns / type

Hello,  can anyone explain why when using transform columns if the function is used on it's own, the declared column type also changes, but if the full syntax is used, it just the column values are...
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  • m_dekorte's avatar
    m_dekorte
    4 months ago

    Hi Dicken,

     

    When you write:

    Table.TransformColumns(Source, {"anumber", Number.From})

    you are passing Power Query the built-in Number.From function directly. Because that function has a known return type of number, Power Query can often detect that and update the column’s type automatically.

     

    When you write:

    Table.TransformColumns(Source, {"anumber", each Number.From(_)})

    you are creating a new inline anonymous function. Even though it calls Number.From, that function itself is untyped. As a result, only the values are converted.

     

    To ascribe type explicitly when using each, pass the new column type as third argument:

    Table.TransformColumns(Source, {"anumber", each Number.From(_), type number})

     

    When you write:

    ascribeNum = (x) as number => Number.From(x),
    transform = Table.TransformColumns(Source, {"anumber", ascribeNum})
    you are creating a new function with a return type of number. Like when passing the built-in Number.From function directly, Power Query can often detect that and update the column’s type.

     

    I hope this is helpful.

  • Ahmedx's avatar
    Ahmedx
    4 months ago

    In the third parameter you can specify the column type
    Why does Power Query behave this way? Is it because of "lazy evaluation"

Mohamed32's avatar
Mohamed32
Icon for Advocate III rankAdvocate III
4 months ago

Here’s a Great question — this behavior in Power Query can be a bit confusing at first.

 

When you use Transform Columns with a function only (e.g. each Text.Upper(_)), Power Query may automatically re-evaluate and update the column’s data type based on the result of that function. This happens because Power Query tries to infer the new type after transformation.

 

However, when you use the full syntax and explicitly define the type (e.g. {"ColumnName", each Text.Upper(_), type text}), you are telling Power Query to apply the transformation but keep (or enforce) a specific data type.

 

So the difference comes down to:

 

Without type → Power Query infers the resulting data type

 

With type → You control the resulting data type explicitly

 

 

That’s why in the second case only the values change, while the data type remains consistent as expected.