Почему SQL-сервер генерирует хранимые процедуры с помощью sp_executesql оператор со строкой?

использовать объединение с таблицей клиентов и подзапросом

SELECT subquery2.customer_trn, c.customer_name   ,avg(balance) FROM 
  ( 
  SELECT depositor.customer_trn, account.account_number, account.balance FROM 
    account 
      INNER JOIN savings_account ON account.account_number = savings_account.account_number 
      INNER JOIN depositor ON depositor.account_number = account.account_number 

    UNION ALL 
  SELECT depositor.customer_trn, account.account_number, account.balance FROM 
    account 
      INNER JOIN checking_account ON account.account_number = checking_account.account_number 
      INNER JOIN depositor ON depositor.account_number = account.account_number 
  ) 
  AS subquery2 join customer c on c.customer_trn =subquery2.customer_trn 

GROUP BY subquery2.customer_trn,c.customer_name ;