Stored Procedure WHERE OR HAVING Does Not Work - mysql

Question

In the below procedure,
It gives error ([Err] 1111 - Invalid use of group function) when I use WHERE statements and it gives another error ([Err] 1054 - Unknown column 'roomid' in 'having clause') when I use HAVING statements instead of WHERE, although I have the roomid column defined in the table.
Does anybody have an idea on how can I fix this issue? The code was working perfectly when I tried under another mySQL version I guess.
DELIMITER //
DROP PROCEDURE IF EXISTS `findAVG`//
CREATE DEFINER=`kamer`#`%` PROCEDURE `findAVG`(IN rid INT, startDate DATETIME, OUT Score DOUBLE)
BEGIN
DECLARE finishDate DATETIME;
DECLARE DateIterator DATETIME;
DECLARE avgPrice DOUBLE;
SET avgPrice = 0;
SET Score = 0;
SELECT price into Score
FROM bookings
WHERE roomid = rid
ORDER BY ABS( DATEDIFF( bookings.date, startDate)) LIMIT 1;
IF (Score = 0) THEN
SET DateIterator = startDate;
SET finishDATE = DATE_ADD(startDate, INTERVAL 30 DAY);
WHILE DateIterator <= finishDate DO
SELECT price
INTO avgPrice
FROM prices
WHERE roomid = rid
AND date = DateIterator
ORDER BY DATEDIFF(startDate, prices.timestamp) LIMIT 1;
SET DateIterator = DATE_ADD(DateIterator, INTERVAL 1 DAY);
END WHILE;
SET Score = AVG(avgPrice);
END IF;
UPDATE bookings SET price=Score WHERE roomid= rid AND date=startDate;
END

Answer 1

"Invalid use of group function" mean you've used a column in the group by function that does not appear in the select clause or the inverse.
i.e. :
SELECT A,B,C,COUNT(*) FROM mytable GROUP BY A,B,C
Grouping is done on the 3 firts's columns and the count is done on each line where A,B and C are similar.
For example, if you try to pass "GROUP BY A,B" SQL wont know what to do with C column...
For HAVING problem, it seem to me the HAVING clause is applicated after the select have been done.
So if you're using a column that doesn't appear in the SELECT clause, then it raise an error.