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MyQuery1.SQL.Text := 'select id,name,0.00 as saldo from Customers';
MyQuery1.Execute;

While not MyQuery1.Eof do

Begin
MyQuery1.Edit;
MyQuery1.FieldByName ('saldo').AsFloat := 20;
MyQuery1.Next;
End;

The version 5.10 it works correctly.
The version 5.20 the following error
//#42522 Unknown column saldo in field list

Verify if the table 'Customers' has field 'saldo'. If it has not, then the error arises because on assigning a value to this field TMyQuery tries to save it in the database.
If you want to use this field as local, set the TMyQuery.Options.SetFieldsReadOnly property to True, and then for 'saldo' field the ReadOnly property to False.
Use MyDAC version 5.20.1.14.

I did that, but you tell me that the field cannot get modified.
I pay out the field not this at the table, it is a virtual field.

You should use the following code: If you set the SetFieldsReadOnly property to True then 'saldo' field won't be included to queries of updating table. As for this it is set as read only.
In order to use it as a variable virtual field set ReadOnly to false.

Thanks