Using SELECT with UNION - sql

Question

CREATE TABLE members
(
name varchar(60),
ID char(6) PRIMARY KEY
);
CREATE TABLE ratings
(
memberID char(6) REFERENCES members(ID),
rating SMALLINT CHECK(rating >= 1 AND rating <= 8),
gameID integer REFERENCES games(ID),
PRIMARY KEY (memberID, gameID)
);
Hi guys, I'm trying to list all the members who have rated a game or not with showing the Max rate, Min rate,average rate, and how many times they have rated.
I tried :
(SELECT MAX(rating), MIN(rating), AVG(rating), COUNT(rating), name
FROM ratings, members
WHERE ratings.memberID = members.ID
GROUP BY name)
UNION
(SELECT MAX(rating), MIN(rating), AVG(rating),COUNT(distinct rating), name
FROM ratings, members
WHERE
members.ID NOT IN (SELECT memberID
FROM ratings, members
WHERE ratings.memberID = members.ID)
GROUP BY name);
This first part gives me a correct values; it gives the correct names followed by Max, min, and count, and the average. But the second part gives a correct names but wrong values of Max, Min, Average. It gives a Max of 9 and Min of 2 for all members who didn't rate any game! Which is not true. How can i fix the second part , so it gives a value of zero instead of 9 and 2 ?

Answer 1

I think you can get the result set you are looking for by using either a LEFT or RIGHT join
SELECT
M.name,
MAX(rating),
MIN(rating),
AVG(rating),
COUNT(rating)
FROM
[members] M
LEFT OUTER JOIN
[ratings] R
ON
M.ID = R.memberID
GROUP BY
M.name
This would then give a results like
name max | min | average | count
name1 8 | 2 | 5 | 3
name2 NULL | NULL | NULL | 0

Answer 2

I'd rewrite that query using INNER JOIN syntax, as it is clearer why the joins bring back unexpected results. As you'd written it, the second query would join all results against all members that had no results, per:
SELECT MAX(rating), MIN(rating), AVG(rating),COUNT(distinct rating), name
FROM ratings, members
WHERE
members.ID NOT IN (SELECT memberID
FROM ratings, members
WHERE ratings.memberID = members.ID)
I suspect you were after something more like:
SELECT MAX(rating),
MIN(rating),
AVG(rating),
COUNT(rating),
name
FROM ratings
INNER JOIN members
ON ratings.memberID = members.ID
GROUP BY name
UNION
SELECT MAX(rating),
MIN(rating),
AVG(rating),
COUNT(distinct rating),
name
FROM ratings
WHERE ratings.memberID NOT IN (SELECT memberID FROM members)
GROUP BY name