docs: add overwrite_src_cols documentation for derived_columns

Documents the new per-column boolean parameter in macros/staging/stage.sql
(derived_columns_description) and in the derived-columns markdown guide,
including a umlaut-rename example and an OVERWRITE SOURCE COLUMNS section.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pkumar-data

docs/01_macro-instructions/02_staging/04_derived-columns.md

@@ -24,6 +24,11 @@ derived_columns:
         src_cols_required:
             - <src_col_2>
             - <src_col_3>
+    <col_alias_3>:
+        value: <expression_3>
+        datatype: <datatype_3>
+        src_cols_required: <src_col_3>
+        overwrite_src_cols: true   # optional — drops src_col_3 from the output
 ```
 
 Depending on how `col_alias` and `src_col` are called, two different behaviors can be achieved:
@@ -51,3 +56,20 @@ It must only be set for SQL expressions. For static strings, it will be set to S
 The parameter `src_cols_required` is only required when the Stage model is configured to not include the source columns by setting the parameter `include_source_columns` to false.
 
 If this is the case, you have to list all columns used within the SQL expressions under the parameter `src_cols_required`. This information is required to properly generate the model SQL.
+
+## OVERWRITE SOURCE COLUMNS
+
+The optional boolean parameter `overwrite_src_cols` can be set per derived column. When set to `true`, the source columns listed under `src_cols_required` are **excluded from the staging CTE output**, so only the derived alias column appears — preventing duplicate columns.
+
+This is particularly useful when **renaming columns whose original name cannot be referenced downstream**, for example columns containing special characters (e.g. German umlauts like `ä`, `ö`, `ü`) on platforms such as Databricks. Without `overwrite_src_cols: true`, both the original and the renamed column would appear side by side in the output.
+
+```jinja
+derived_columns:
+    account_name_clean:
+        value: Kontonäme
+        datatype: STRING
+        src_cols_required: Kontonäme
+        overwrite_src_cols: true
+```
+
+In this example, `Kontonäme` is excluded from the staging CTE and only `account_name_clean` appears in the output. The parameter defaults to `false` when omitted.

macros/staging/stage.sql

@@ -53,9 +53,14 @@
   " %}
 
   {% set derived_columns_description = "
-    derived_columns::dictionary         Defines values and datatypes for derived ('added' or 'calculated') columns. The values of this dictionary are the desired column names,
-                                        the value is another dictionary with the keys 'value' (holding a column name, a SQL expression, or a static string beginning with '!') and
-                                        'datatype' (holding a valid SQL datatype for the target database).
+    derived_columns::dictionary         Defines values and datatypes for derived ('added' or 'calculated') columns. The key of each entry is the desired output column name.
+                                        The value is a dictionary that supports the following keys:
+                                          'value'            (required) A column name, a SQL expression, or a static string beginning with '!'.
+                                          'datatype'         (required) A valid SQL datatype for the target database.
+                                          'src_cols_required' (optional) A column name or list of column names used by the expression. Required when include_source_columns=false.
+                                          'overwrite_src_cols' (optional, default false) When true, the columns listed in 'src_cols_required' are excluded from the staging CTE
+                                                             output, so only the aliased column appears. Useful for renaming columns whose original name cannot be referenced
+                                                             downstream (e.g. columns with special characters such as umlauts on Databricks).
 
                                         Examples:
                                             {'conversion_duration': {'value': 'TIMESTAMP_DIFF(conversion_date, created_date, DAY)',     Creates three derived columns. The column 'conversion_duration' calculates
@@ -64,6 +69,11 @@
                                                                      'datatype': 'STRING'},                                             The column 'account_name' duplicates an already existing column and gives
                                              'account_name':        {'value': 'name',                                                   it another name. More derived columns can be added as additional keys of
                                                                      'datatype': 'String'}}                                             the dictionary.
+
+                                            {'account_name_clean': {'value': 'Kontonäme',                                              Renames a source column whose name contains a special character. Setting
+                                                                    'datatype': 'STRING',                                              'overwrite_src_cols: true' ensures the original column is dropped from the
+                                                                    'src_cols_required': 'Kontonäme',                                  CTE output, so only 'account_name_clean' appears and the special-character
+                                                                    'overwrite_src_cols': true}}                                       column name does not propagate downstream.
   " %}
 
   {% set sequence_description = "