Cloud SQL to BigQuery Upsert Query using external connection

I recreated your issue and here's the solution where I have written a procedure to take values dynamically from Cloud SQL using EXTERNAL_QUERY() and insert or update based on conditions.

CREATE OR REPLACE PROCEDURE `<YOUR_BIGQUERY_DATASET>.LoadDataFromCloudSQL`()
BEGIN
  DECLARE sql_query STRING;
  DECLARE column_names STRING;
  DECLARE merge_statement STRING;
  DECLARE transformed_columns STRING;


  
  -- Retrieve data from Cloud SQL using EXTERNAL_QUERY()
  SET sql_query = "SELECT * FROM EXTERNAL_QUERY('<EXTERNAL_CONNECTION_NAME>', '''SELECT * FROM <YOUR_TABLE_NAME> WHERE created_at > now() - interval '24 hours';''')";

  -- Get the column names from the source table
  SET column_names = (
    SELECT STRING_AGG(column_name, ', ')
    FROM (
      SELECT column_name
      FROM `<YOUR_BIGQUERY_TABLE>.INFORMATION_SCHEMA.COLUMNS`
      WHERE table_name = '<YOUR_TABLE_NAME>'
    )
  );

  -- Get the columns for update query
  SET transformed_columns = (
    SELECT STRING_AGG(CONCAT(column, ' = source.', column), ', ')
    FROM UNNEST(SPLIT(column_names, ',')) AS column
  );


  -- Generate the MERGE statement dynamically
  SET merge_statement = CONCAT('''
    MERGE `<YOUR_BIGQUERY_TABLE>` AS target
    USING (
      ''', sql_query, '''
    ) AS source
    ON target.id = source.id
    WHEN MATCHED THEN
      UPDATE SET ''',transformed_columns,'''
    WHEN NOT MATCHED THEN
      INSERT (''', column_names, ''')
      VALUES (''', column_names, ''')
  ''');

  -- Insert new rows and update existing rows in BigQuery table
  EXECUTE IMMEDIATE merge_statement;

  -- Output the completion message
  SELECT 'Data transfer completed.' AS result;
END;