我一直收到错误ORA-01422，ORA-06512

Question

我一直收到错误ORA-01422：确切的提取返回超过请求的行数。 ORA-06512：at＆＃34; CM420A07.ZIP_CODE_SALES_WINNER＆＃34;，第6行。我无法确切地知道出了什么问题。

CREATE OR REPLACE FUNCTION ZIP_CODE_SALES_WINNER
  RETURN VARCHAR
IS
  zipWinner VARCHAR2(5);
BEGIN
  SELECT C.zip_code INTO zipWinner
  FROM SALE S JOIN CUSTOMER C
  ON S.cust_ID = C.cust_ID
  GROUP BY C.zip_code
  HAVING COUNT(*) = (
  SELECT MAX(COUNT(*))
  FROM SALE S JOIN CUSTOMER C
  ON S.cust_ID = C.cust_ID
  GROUP BY C.zip_code);  
  RETURN zipWinner;
END;
/
SELECT ZIP_CODE_SALES_WINNER FROM DUAL;