mysql 函数问题

请问mysql的自定义函数怎么建啊。可不可以不要返回值，可不可以向存储过程那样就执行一段语句，不要返回值。下面这段存储过程怎么样转成自定义函数。
create procedure tran()
BEGIN
  DECLARE ID VARCHAR(40);
  DECLARE ID1 VARCHAR(40); 
  DECLARE DT VARCHAR(40);
  DECLARE done INT;
  DECLARE Y VARCHAR(40);
  DECLARE M VARCHAR(40);
  DECLARE R VARCHAR(40);
  DECLARE TEMP VARCHAR(40); 
  DECLARE DT1 VARCHAR(40);
   

  DECLARE CUR_TRAN CURSOR FOR SELECT MEMBERID,PRODUCTID,date from reviewsnew;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done=1;
   
  OPEN CUR_TRAN;
   
  REPEAT FETCH CUR_TRAN into ID,ID1, DT;
  SET DT1=DT;
  SET Y=SUBSTRING_INDEX(DT,',',-1);
  SET M=SUBSTRING_INDEX(DT,' ',1);
  SET TEMP=SUBSTRING_INDEX(DT,',',1);
  SET R=SUBSTRING_INDEX(TEMP,' ',-1);
   
IF M like 'January' THEN
  SET TEMP=concat(Y,'-','01-',R);
END IF;
IF M like 'February' THEN
  SET TEMP=concat(Y,'-','02-',R);
END IF;
IF M like 'March' THEN
  SET TEMP=concat(Y,'-','03-',R);
END IF;
IF M like 'April' THEN
  SET TEMP=concat(Y,'-','04-',R);
END IF;
IF M like 'May' THEN
  SET TEMP=concat(Y,'-','05-',R);
END IF;
IF M like 'June' THEN
  SET TEMP=concat(Y,'-','06-',R);
END IF;
IF M like 'July' THEN
  SET TEMP=concat(Y,'-','07-',R);
END IF;
IF M like 'August' THEN
  SET TEMP=concat(Y,'-','08-',R); 
END IF;
IF M like 'Septemper' THEN
  SET TEMP=concat(Y,'-','09-',R);
END IF;
IF M like 'October' THEN 
  SET TEMP=concat(Y,'-','10-',R);
END IF;
IF M like 'November' THEN
  SET TEMP=concat(Y,'-','11-',R);
END IF;
IF M like 'December' THEN
  SET TEMP=concat(Y,'-','12-',R);  
END IF;

update reviewsnew set date=TEMP WHERE MEMBERID=ID AND PRODUCTID=ID1 AND date=DT1;
   
  until done=1 end repeat;
   
CLOSE CUR_TRAN;
END

有返回值你不用不就行了吗