Yes, this query works fine. Thank you for your help. Also, I found one more solution by using Power Query.

Solution 1. SQL Function

First, create the function. Unfortunately, Fabric doesn't support (yet?) scalar functions, so I used table-valued one

CREATE FUNCTION dbo.Base64ToUUID (@base64String VARCHAR(MAX))
RETURNS TABLE
AS
RETURN(
SELECT cast(base64_decode(@base64String) AS UNIQUEIDENTIFIER) AS UUID
);

Then the function can be used as Ad-hoc transformation in queries:

select 
 "_id.$binary.base64" AS Original,
  U.UUID AS Converted
from dbo.Data c
CROSS APPLY dbo.Base64ToUUID("_id.$binary.base64") AS u

or it can be used in Dataflows Gen2 to transform the data and store:

add the function under Get Data

let
  Source = Fabric.Warehouse(null){[workspaceId = "bf28769e-c926-427d-b860-2e9999fe2fea"]}[Data]{[warehouseId = "33539h44-9bfc-48dd-93ba-73b0898ad6a9"]}[Data],
  #"Navigation 1" = Source{[Schema = "dbo", Item = "Base64ToUUID"]}[Data]
in
  #"Navigation 1"

then in other query use it:

// transformation
  Result = Table.TransformColumns(Data,
  {
    {"Id", each Base64ToUUID(_)0}[UUID], type nullable text},
     ...
  }),

Solution 2. Power Query

let
  ...
  Result = Table.TransformColumns(Data, {
    {"_id.$binary.base64", each GetGuid(_), type nullable text}
  }),

  GetGuid = (guidBase64 as nullable text) as nullable text =>
    if guidBase64 = null then null
    else
    let
        binaryGuid = Binary.FromText(guidBase64, BinaryEncoding.Base64),
        textGuid = Text.FromBinary(binaryGuid),
        guid = Guid.From(Binary.ToText(binaryGuid, 1))
    in 
      TransformGuid(guid),
  TransformGuid = (guid as text) as text =>
    let
      parts = Text.Split(guid, "-"),
      transformedParts = List.Transform(parts, each SwapLetterPairs(_)),
      finalGuid = Text.Combine({transformedParts{0}, transformedParts{1}, transformedParts{2}, parts{3}, parts{4}}, "-")
    in 
      finalGuid,
  SwapLetterPairs = (word as text) as text =>
    let
      pairs = List.Generate(
        () => [i = 0, result = ""],
        each [i] <= Text.Length(word),
        each [i = [i] + 2, result = Text.Range(word, [i], 2) & [result]],
        each [result]
      )  
    in
      List.Last(pairs)
in
  Result

here you can see a little bit of magic. I'll try to explain. I don't know why but this Power Query code can return a UUID but the first part of that UUID has swapped symbols pair! Example:

guidBase64 = "zV4m6kZt80q0HXb9ERVFdA==" // which represents ea265ecd-6d46-4af3-b41d-76fd11154574
binaryGuid = Binary.FromText(guidBase64, BinaryEncoding.Base64),
textGuid = Text.FromBinary(binaryGuid),
guid = Guid.From(Binary.ToText(binaryGuid, 1))
// guid is : cd5e26ea-466d-f34a-b41d-76fd11154574
// Original: ea265ecd-6d46-4af3-b41d-76fd11154574
// problem is  here   here here

So I defined the SwapLetterPairs function.

Is it a bug or is it by design? It would be great to have a simpler solution but ...

I use Solution 1 because it is more native and less error-prone. Hope this will be helpful for those who have spent hours trying to get this to work 😉

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